does it raise a move by 3 or somthing???
It maxes out your PP for example a move with 5 pp can be raised to have a max of 8 pp. Unlike pp ups, which it takes several to max out the pp, pp maxes, hence their name, do it in one move
It automatically raises a move's PP to the move's maximum amount, as if you used 3 PP Ups in one. For example, if Curse had 10 PP, the PP Max would raise it to 16.