In tennis, a Let is called when a point has been interrupted in some way. The most common example of a Let is when the ball catches the net or a serve, the umpire calls a Let and the player retakes the serve without losing the point.
A Let can also be called for other reasons during a point.
This includes if a ball rolls onto the court or there is a loud noise that distracts the players. When a let is called, the whole point is replayed from the beginning.
They are a common part of the game and several famous players including Roger Federer and Serena Williams have had Lets called on them.