To powerlevel is, in role-playing games or RPGs (nearly always in MMORPGs), to quickly level up a lower-level character by killing high-level mobs with a higher-level character. To do this, first have one (or more) high-level character(s) and one (or more) low-level character(s). Group them together. Go to a high-level area or zone, leaving the low-level character(s) at the entrance. Then go out and kill things with the high-level character(s). The low-level character(s) will get much more experience than when killing lower-level mobs. Powerleveling is generally frowned upon by most gamers, as it is the the 'easy way out'.
Many games have eliminated this by creating a system where there is a level range for getting experience in groups. I.E., a level five character cannot get experience grouping with a level fifty character, but a level forty-five character could. Other games (City of Heroes) have a limit to how far you can be from the leader of the group to get experience. In yet other games (Maple Story), monsters have levels, and the level of the character killing the monster has to be within a certain range of the monster's level.