In tennis, to "serve" a ball means to initiate a point by striking the ball with a racket to hit it into the opponent's service box. The serve is crucial because it sets the tone for the rally. Players use various techniques like the flat, topspin, and slice serves to gain an advantage over their opponent. A strong serve can lead to easy points, while a well-placed serve forces opponents into a defensive position, making it a fundamental skill in the game.