Destiny 2 dungeon is a 3 man PVE activity in the game. Prophecy, Pit of Heresy, and Shattered Throne. These are like many raids that can be looted for weapons and armor. Sometimes there is even an exotic tied to the dungeons. They also feature bosses and mechanics that require a skilled team with coordination to complete.
They can also be done solo and flawless for exclusive emblems and other rewards. The master difficulty has been confirmed to be coming soon for even more loot. Like raids, these run on a weekly reset where they can only be looted once per week for each character.
1 New dungeon will be coming in the 30th-anniversary pack. 2 more will be released in the Witch Queen so stay tuned.