PP is an abbreviation of Power Points. Power Points measure the number of times a move can be used. The higher the PP, the more times you can use the move. PP are essential for attacking, supporting, stalling, recovering, and defending. The PP is reduced by one everytime you use a move or can be reduced by a greater number if the opponent Pokémon has the ability Pressure. They are also significant for moves like Trump Card, Fury Cutter, and some others, as well as the Metronome item (based on consecutive usage of the same move). The move Spite will reduce the PP of the last move your opponent used, and vice versa.
Generally, a move with lower PP is much stronger than a move with higher PP. Though these moves can also have downsides to them such as causing recoil, negative stat changes and while having a higher attack power have less accuracy to them as well.