What is topspin in tennis?
The spinning ball drags air unevenly, which pushes it downward: the Magnus effect. That lets you hit higher over the net and still land inside the baseline, which is why topspin is the margin shot.
You make it by brushing up the back of the ball with a low-to-high swing and a near-vertical racquet face.
Questions
Why does topspin make the ball dip?
The spin creates a downward Magnus force, so the ball curves down faster than gravity alone would pull it.
Is topspin safer than a flat shot?
Usually. A heavy topspin ball can clear the net by more and still land in.