The Light, also known as the Gardener or the Sky is a paracausal force associated with the Traveler. Its counterpart is the Darkness, which it has been at war with since before the beginning of time.
Like the Darkness, the Light is a paracausal force that has existed since before the beginning of time and space. It is fundamentally driven to increase the complexity and diversity of entities within the universe, and in particular to encourage the growth of new lifeforms.
The Light emanates from the Traveler, but it is not exclusively produced by it. All living things and all places possess some Light, —former possibly as a soul— as the spark of life is Light in itself. Ulan-Tan hypothesized that all Light is connected, later confirmed when Guardians discovered the Hive siphoning light from the Traveler through a piece of itself. The Light is antipathic to the Hive, though they are known to feed on it and offer it as a sacrifice to their gods.
Guardians are the rare few able to wield the Light as a weapon. Additionally, born from the Traveler itself, the Light was used to construct the Ghosts. Orbs of Light may also be found, which give power to a Guardian's Super, their ultimate use of the Light. Light, as used by the Guardians, comes in three different varieties; these are the elemental damage modifiers, and are used in both a Guardian's chosen subclass and in certain weapons. It is possible for Light to be corrupted, as the shard of the Traveler within the European Dead Zone is corrupting an area of the surrounding forest. In addition, the Light Dominus Ghaul wielded was corrupted due to him taking Light from the Traveler, which was an act of Darkness.
In addition to being a tangible paracausal force, the Light is also synonymous with a certain existential philosophy, as detailed in several Lore entries. The way of the Light emphasizes cooperation between intelligent beings, with the goal of creating "a gentle world" where all entities may harmoniously coexist. Its antithesis is the philosophy of the Darkness, which claims that "existence is the struggle to exist" and that all intelligent life must compete to survive at the expense of other beings. The philosophy of the Light is also referenced in the Marasenna as "the way of the bomb," meaning that it encourages a complex world with many interrelated parts that rely on each other to achieve greater goals. In contrast, the philosophy of the Darkness is described as "the way of the sword", which seeks to reduce everything in the universe to a final, simplified, self-same state.
According to the Lore Book "Unveiling," which is seemingly narrated by the Darkness itself, both the Light and the Darkness have existed since before the start of time, and thus before the universe came into existence. Being unbound by the laws of causality, neither the Light or Darkness can be said to have a beginning. Rather, they are emergent properties of even more fundamental "mathematical structures" that in turn underlay reality itself. 
In the allegorical story recounted in the Lore Book, the Darkness and Light occupied themselves by engaging one another in a game, analogous to a vastly more complex version of the "Game of Life" devised by Earth mathematician John Conway. This game would consistently arrive at an end-state dominated by a single, self-perpetuating pattern, which subsumed all others in the game. The Darkness found this to be a pleasing outcome, but the Light felt the pattern to be boring, and desired that the game endlessly produce novel patterns instead. In order to promote this novelty, the Light transformed itself into a new "rule" within the game; the Darkness did the same, to counteract the Light's efforts. These new rules were "set aside" from the other rules of the game, so that they could not be limited or influenced by them, but could manipulate the other rules to bring about their intended outcomes.
While the story above is laden with metaphor and open to interpretation, it appears to suggest that the universe is the "game" which the Light and Darkness played in the garden, that the rules of the game are the laws of physics as they are commonly understood, and that the "new rules" that the Light and Darkness became are the basis of paracausality.
The Ancient Past
The earliest recorded mention of the Light comes from the Books of Sorrow, a collection of ancient Hive texts chronicling the origin of the Hive and their history of interstellar conquest. The Light is referred to as "The Sky," and is described as a philosophy that upholds collaboration and gentleness with the goal of creating a thriving, harmonious society and universe. Its counterpart, "The Deep," is also described, being a philosophy claiming that existence is the struggle to exist, and it is the goal of all life to challenge and defeat other life so as to become stronger and continue existing. Several disciples of the Light, including the Leviathan and the Traveler, also feature in these texts.
Before the rise of the Hive, the Traveler had visited several civilizations and bestowed the blessings of the Light. These civilizations included the Ammonite and the Harmony. At an unknown time in the past, the Traveler also came to the Eliksni and uplifted their civilization.
The Golden Age
When the Traveler came to the Sol System, it used the Light to terraform several planets and moons to render them suitable for human habitation.
When the Darkness pursued the Traveler to the Sol System, a cataclysmic battle ensued in which the Traveler used the light to push back the Darkness, at the cost of rendering itself largely inert. Fragments of the Traveler were thrown far from the site of the battle, with one of the largest landing in Europe. The Light emanating from this shard interacted with the surrounding landscape, creating the European Dead Zone.
While it battled the Darkness on Earth, the Traveler also extended a ray of Light to protect the distant colony ship Yang Liwei from the Darkness. This ray pushed back the Darkness, but could not repel it entirely; the energy of the resulting struggle was so great that a kugelblitz formed, which swallowed the Yang Liwei and its crew.
The City Age
After its sacrificial last stand, the Traveler created the Ghosts, small machines infused with Light, to seek out long-dead individuals and resurrect them as warriors with Light-based abilities. These warriors, first called Risen and later known as Guardians, can manifest weapons and defenses made of Arc, Solar or Void Light, and can return from death with the assistance of their Ghost.
During the events of Destiny 2, the Cabal Red Legion invaded the Solar System and attacked the Last City. Forces under Dominus Ghaul successfully captured the Traveler, and placed a cage around it, thus cutting all Guardians connection to the light. Yet, at least one Guardian managed to reclaim their light from a corrupted Shard of the Traveler in the European Dead Zone, proving that not all light emanated from the Traveler itself. Ghaul attempted to repeatedly gain the Traveler's favor and grant his Red Legion the ability to wield the light. However, the Traveler refused, and Ghaul resorted to outright stealing the light by siphoning it from the Traveler via its cage. Light was only restored to all Guardians after the Traveler broke free of its cage and destroyed once and for all a power-hungry Ghaul.
In Forsaken, it is revealed that the Traveler had been "speaking" to the Guardians through visions of crowns, roses, silver trees, and torches turning into bonfires since its awakening. Ikora Rey tells the Guardians to follow these visions, which leads them to Io. The remnant light on Io then granted the Guardians new powers.
During Year One of Destiny, Light was a value present on all level 20 armor of Rare, Legendary, and Exotic rarity. In order to increase a character's level beyond 20, the player had to obtain armor with more and more Light, which would give them a Light level between 21 and 34 depending on the Light value of the gear. The highest attainable Light level in Year One (after the release of the House of Wolves expansion) was level 34, which required four armor pieces (one for each armor slot available at the time) with 36 Light each, which came to a total of 144 Light.
Light level determined the player's strength and what activities the player could participate in. To participate in an activity effectively, players had to have a Light level that matched or exceeded the activity's recommended Light level, or the higher-level enemy units would be able to both inflict significantly more damage on the player and resist more of the player's attacks. If the player was as little as four levels below an enemy, the player's damage output would be reduced to the point that the enemy would be "immune" to the player's attacks; the player would actually still be damaging the enemy, but the amount of damage being inflicted would be so small as to render killing the enemy within a reasonable amount of time an extremely difficult, if not impossible, feat.
The Taken King changed the way Light was determined and used by players. Light is now a weighted average of the Attack and Defense values of all of the player's currently equipped gear; weapons count more heavily in the calculation than armor. Under this new system, Light reflects the player's strength and what activities the player is prepared for, but operates on a less severe penalty curve than Year One Light levels; whereas a Light deficit of four would make the player harmless to an enemy in Year One, in Year Two it takes a much higher Light deficit to noticeably impair the player's combat effectiveness. The Attack and Defense values of all Year One gear is capped at 170, meaning a player must equip Year Two gear in order to reach a higher level of Light.
Certain thresholds of Light must be attained in order to unlock or be able to complete some activities: Light 240 unlocks the Daily Heroic Story, Light 260 unlocks the Vanguard Heroic Strikes playlist, and Light 280 unlocks the Nightfall. The Raid King's Fall is available immediately, but it is recommended to achieve a Light level of 290 before attempting. The highest attainable Light level in Year Two was 335.
The Light mechanic remained the same as Year Two with the transition to Rise of Iron, but the limit was raised. Light 300 unlocks the Daily Heroic Story, and Light 320 unlocks the SIVA Crisis Heroic Strikes playlist and the Nightfall. The Raid Wrath of the Machine will have a recommended Light level of 360 on normal difficulty, and 385 on heroic difficulty. The highest attainable Light level in Year Three was 400.
The Light mechanic from Year Two of Destiny was carried over to Destiny 2, but was renamed Power. Everyone starts at 750 Power, and can raise their natural Power up to a maximum of 970 with the introduction of the Season of Dawn, then augment it further by leveling up the seasonal artifact. However, soft caps make reaching 970 power with gear difficult. While all engrams will raise a player's average Power level until they reach Power level 900, and Prime Engrams, Exotic Engrams, and powerful rewards will take them to 960, only pinnacle rewards will be able to level a player from 960 to 970. There are only a few sources of pinnacle rewards, each of which can only be acquired once per week, and those are:
Most content in Destiny 2 currently has a recommended Power level of 750, effectively meaning they have no Power requirement. The exceptions are content coming with and following Shadowkeep, such as its campaign, Nightmare Hunts, the Pit of Heresy, the Garden of Salvation, and the Sundial. Additionally, Nightfalls can impose a Power handicap through the Five of Swords (purchasable from Xur), and Ordeals, Nightmare Hunts, and the Sundial all have multiple difficulties, ranging from 750 Power to 980 Power (among other differences).
List of appearances