From Destinypedia, the Destiny wiki
A Flashpoint is a weekly challenge previously given by Cayde-6 in Destiny 2. With the introduction of Update 2.0.1 and Forsaken, Flashpoint rewards are now handed out by the vendor at the chosen Destination.
One Destination is chosen for Guardians to focus their efforts on by completing Public Events, Lost Sectors and Heroic Adventures. Participating in a Public Event but not completing the objective before the timer runs out grants 7% towards the Flashpoint; completing a normal Public Event grants 15%; completing a Heroic Public Event grants 20%. Completing the Flashpoint rewards a Powerful Engram, which can drop Nightfall-quality loot and always guarantees a piece of gear with a Power level at or above a player's level.
While a Flashpoint is active at a Destination, additional enemy lieutenants appear in certain areas for brief periods, often in the middle of Public Events. Killing these enemies is entirely optional and does not grant any additional percentage towards Flashpoint completion.
List of appearances