Use this tag when asking about the target numbers rolls are trying to achieve, in systems which calls that the "difficulty class." (Questions about RPG systems that describe the target number of a roll as the "difficulty", such as Fate, should be tagged [difficulty] instead.)
When games ask us to roll dice, usually we're trying to hit or exceed a target number in order to succeed (or otherwise indicate the outcome's nature). Some systems --notably the d20 System, including D&D 3.x, 4e, 5e, and Pathfinder-- call this target number the "difficulty class." It is often abbreviated "DC."
In some systems these target numbers are called something else (fate calls it the "difficulty", for example). If the system you're asking about uses a different term, tag your question with that system's name for the thing (e.g. difficulty).