PP is an abbreviation of Power Points. Power Points measure the number of times you can use a move. The higher the PP, the more times you can use the move. PP are essential for attacking, supporting, stalling, recovering, and defending. They are also significant for Trump Card, the Metronome item (based on consecutive usage of the same move), Fury Cutter, and some others.
Generally, a move with lower PP is better than a move with higher PP.