There is a small area where the player is allowed to stand when serving the ball in a match. They must stand behind the baseline and in between the center and sideline on the side opposite of the service box where they need the ball to land. A foot fault occurs when a player does any of three things: steps on or over the baseline before striking the ball on their serve, their feet touch or cross the wrong side of the center mark, or their feet cross the wrong side of the extended sideline.
A foot fault is treated the exact same way to any other fault that could occur in a tennis match. It is only counted as a single fault, giving the player a second chance to get the ball over the net successfully.
A foot fault is one of those rules in sports that many professional players do not agree with and can become angry when it is used against them. This can be seen when Serena Williams committed a foot fault during her 2009 US Open semi final match. She got called for a foot fault during the match and she became angry with the line judge because she did not believe that it was necessary to make this call. This rule is often looked over when professionals are serving as it does not give any true advantage to the individual that is serving.