An umpire is the person who says "let" in professional tennis. In a casual match-up, the participants can either appoint a third person or agree that the call should be made by both players. In a casual match, you may also set a "no-let" rule, which means that no lets can be called throughout the game. This may result in a faster match. In any case, let me make this clear: an umpire is not the person who tells you where to hit the ball; they judge whether a player has infringed any of the rules during a point. The word "let" is used in tennis to indicate when a player has broken a string on his racquet. The referee will then ask the player if he wants to take a replacement string from the box or use a new one. If the player chooses to use a new one, the match is over because he has forfeited the point. If he takes a replacement string, the game continues as normal except that he cannot break any more strings. Replacing a string takes about 30 seconds, so this isn't a big problem for the player. As I have already said, an umpire is not the person who tells you where to hit the ball; they judge whether a player has infringed any of the rules during a point. For example, if a player is standing too far back behind the baseline, the umpire will call it out. If a player is not standing up straight, the umpire will call it out.