Forsaken is the biggest expansion in Destiny 2 that changed the game for good.
The journey starts with the Forsaken campaign which is how you start leveling up. Once finished you will then will want to unlock the dreaming city which is available through a quest.
The dreaming city host a bunch of new activities including ascendant challenges, quests, Last Wish raid, 3rd subclass, and the Shattered Throne dungeon. There are new exotic quests like the Wish-Ender, Malfeasance Thunderlord, Thorn, The Last Word, Bad Juju, and more randomly dropped exotics.