In a tennis match, the ball boys pick up any balls that go into the net or off the court and throw them back to the players. In a baseball game, the ball boys are in charge of collecting the balls that are hit out of the field.
ball boy in British English
or ball girl
noun
(in sport, esp tennis) a person who retrieves balls that go out of play
ball boy in American English
Sport
a boy who retrieves balls that have gone out of play, as in tennis and baseball,or a boy in charge of the extra balls used in practice, as in basketball and football