The float serve is one of the types of serves in the game of volleyball. The serve involves hitting the volleyball in such a way as to minimize the rotation of the ball, causing unpredictability in the ball’s movement, and is akin to baseball's knuckleball. During this serve, the ball is moving in the air without spin, allowing drag, lift, and turbulent airflow to have a big effect on its movement. When done correctly, the float serve is hard for opponents to predict where the ball will end up, thus rendering it hard to pass. The effectiveness of the float serve is determined by its ability to succeed in an ace, or winning the point by serving over the net and having the ball hit the ground before the opponent can touch it.

Source: Wikipedia