It means how frequently it hits the opponent. Sometimes a move doesn't hit the opponent, and it will say "The opponent avoided the attack!"
Some moves have 100 accuracy like the move surf. Some have 60 accuracy like hypnosis.These stats can be lowered by certain moves like minimize or double team.
Personally I'd choose Water Pulse due to it's chance of confuseing the opponent and 100 accuracy.everyone thinks the higher power moves are better but they're really not(this is my opinion).
Hope I helped!