What is the purpose of an umpire in tennis?

Answer 1

In tennis, the main umpire is called the chair umpire (they sit in a chair high above the court so they can see the entire court well). In major tournaments, there are usually also line judges who stand behind each line on the court. The line judge calls when a ball is out, and the chair umpire may overrule any line judge's call. The umpire also calls the score after every point.

Answer 2

The chief umpire calls the final decisions on in, out, etc.