A ball boy refers to a kid or person (male and female) that has the task to receive balls from the tennis court and hands them back to the players when they need them.
In the bigger tournaments there are two ball boys at the net (one on each side). And a couple on the backsides of the court.
Normally the two ball boys at the net are responsible for gathering the balls that are out of play. The others are responsible for giving the balls back to the players before they serve.