Since the first raids on Everquest " Adds " refers to the new spawn of minions during a boss fight or encounter. Usually they're there to keep you fighting non stop, you have to deal with them because they have the potential to wipe the raid. WoW did a good job with this on certain fights. A great example is Onyxia. While fighting her you have a swarm of hatchlings appear, not dealing with them ASAP makes the fight much harder ... if not impossible. It's another way for developers to up the difficulty in these areas and extend the boss fight itself. Especially if the boss is inactive during this phase.