Power leveling refers to a higher level healing you while you fight tougher monsters. That way you don't have to rest, you can beat them, and you can keep killing them non-stop.

Bad thing about this, is that new players eventually lose all skill in their jobs and won't party without a power leveler. Very very bad thing. I don't mind PLing older players if they have a job they need to level from 7-10 though.