In DROD, completion comes in many different shapes and sizes!
Every hold released, whether user-made, or official, is conquerable, meaning that there is an accessible stairwell who's destination is the end of the hold. Technically, the player doesn't have to clear any rooms to conquer the hold, however, the vast majority of holds do require rooms to be cleared and enforce this through the use of blue or green gates.
Mastering is when every secret room in a hold is cleared, in most user-made holds this is just for completion's sake, but in the majority of official holds, after mastering the hold, a mastery door is dropped and this gives access to a whole new area of the hold.
Post-mastery is when every room that can only be accessible through mastering the hold is cleared, often, the post-mastery content includes iterations of rooms which did not make the final cut, or easter-eggs like art work by developers.