This phrase is used in football to denote the position of the football prior to the completion of the current play. It is most often used when a penalty is called which requires the ball be moved to a different position. For example, if a play starts at the fifty yard-line and finishes with a pass completion at the ten yard-line, and pass interference is called on the offense, the ball will be moved back to ten yards behind the previous spot, on the forty yard line.
The spot of the ball at the beginning of every down is recorded by game officials. If a penalty is called which requires the ball be moved to the previous spot, or some distance from it, the game officials are called upon to provide the information. The ball must be placed exactly on the previous spot before any additional penalty yards are assessed.