This is an offensive penalty in which one of the front seven moves before the ball is snapped. The offensive team receives a five-yard penalty and replays the down.
This is one of the most commonly called penalties in the game of football. Once a player has set himself he is not allowed to move until the ball has been snapped. Teams such as the Seattle Seahawks credit these types of penalties to fan noise when their teams' defense is on the field. Since 2005 the Seahawks have had more false start penalties called against their opponents than any other team in the league. They will tell you it's the 12th man that causes this to take place.