When a shot has topspin, it creates a downward force that makes the ball fall because of how it interacts with the air (see Magnus effect). In racquet sports, it can be made by hitting the ball with an up-and-forward swing, with the racquet pointing in the opposite direction of where the ball is going.
In tennis, topspin is a shot where the ball rotates forward as it moves through the air. A topspin shot is hit with an upward motion, causing the ball to spin forward. The benefit of hitting a topspin shot is that it makes the ball travel over the net at a higher trajectory than a flat shot or a slice. This makes it more difficult for your opponent to return the ball, and also allows you to hit the ball with more power.
In addition, because the ball drops quickly after hitting the ground, you can hit multiple topspin shots in a row without your opponent having time to recover. As a result, topspin is an essential skill for any tennis player who wants to improve their control over their shots.