Like any other sport, in football if a player commits a penalty, there is a consequence. Loss of a down is one of the consequences that teams face when committing a penalty during a game. Losing a down is only possible for the team that has the ball, and so faults committed exclusively by the offense can lead to the loss of a down. With that, if the offense is in a second-down situation needing to advance 10 yards and commits a penalty, the play will be restarted and the team will find itself in a third-down situation, still needing to travel the same 10 yards.
As if losing a down was not bad enough, some penalties can lead to a combination of losing a down and losing some yards. When committing one of those penalties, a team that finds itself in a second-down situation needing to advance 5 yards has to restart the play a few yards back and in a third-down situation.
The penalization for faults can vary depending on the various levels and football leagues, and that includes the loss of a down. In the NFL, the penalties that most commonly lead to the loss of a down are intentional grounding and illegal pass, which happens when a pass is made after the ball passes the line of scrimmage. Both of those penalties lead to the loss of a down and the loss of 5 yards. When a team loses a down, the official signals it by putting both hands on the back of his head.