1. In a game (particularly multiplayer role playing games), to gain experience--and thus levels--at a greater rate than is normally planned for the game; often accomplished with the help of other characters or with the use of exploits.
2. Playing a game for a long session with the specific intent to gain experience--and thus levels.
The experience is so good here that we're gonna stay and powerlevel.
Derived from Dragon Ball Z, in that the greatest warriors in that anime would have ridiculously high power levels.
In anime-related forums, most notably /a/, it refers to the fact that one watches anime or reads manga. It is usually used in the context of keeping it a secret from others.
Turns out he had a power level too. And then we had sex