From Destinypedia, the Destiny wiki
The "flower game" is a metaphor used to describe the simulated realities the Light and Darkness created together in the primordial "garden" that preceded the universe. It is described by the Darkness as a version of Conway's Game of Life, acted out using flowers that opened and closed to represent shaded and empty squares, respectively. In actuality, the game was vastly more complicated than this allegorical description, being able to describe entire universes. (Conway's Game of Life is Turing Complete and advanced simulation is possible within the confines of its ruleset)
As the Light and Darkness played the flower game, a particular self-sustaining pattern would always emerge which ended up dominating the entire game. The Darkness found this to be a pleasing outcome, but the Light was irritated by it, desiring that the game produce endless novel patterns instead. In order to achieve their desired result, the Light made itself into a new rule within the game that would promote diversity and complexity; the Darkness did the same, becoming a rule that enforced simplicity.
As the Light and Darkness fought with one another, the patterns that had previously dominated their games escaped into the nascent universe, where they eventually became the Vex.