Where the server puts his foot onto or over the Baseline before hitting the ball. If performed on a 1st serve, you will only have your 2nd serve remaining. If performed on your 2nd serve you lose the point.
A server is foot-faulted if one of his feet touches or passes over the end-line while serving. Any other player will have a foot fault called against him if either foot passes the centre-line into the opposing court.
Usually a fault caused by the server's foot entering the court before the racket contacts the ball. It is also a foot fault if any part of the server's foot is on the wrong side of the center mark, or the server is walking or running while delivering the serve. See also serve.