Light on content.
If you simply ran out of things to do in vanilla Destiny 2, its first DLC expansion, Curse of Osiris, adds a few new activities for you to take on. It introduces a new setting in Mercury, a short campaign, new weapons and gear, Strikes, Crucible maps, Adventures, among smaller things.
The Infinite Forest is an interesting concept with some gorgeous location designs; Experimenting with new loot shows off Destiny's fantastic gunplay; The Raid Lair is a fun challenge to take on with a Fireteam
Locks you out of the hardest content if you only have the base game; and that content isn't fundamentally different from before; The campaign rushes through a high-stakes story and feels half-baked as a result; Mercury's activities feel like busywork with no compelling incentive to do them