The line of scrimmage is an imaginary line on a football field that separates the offensive team from the defensive team. It is established at the spot where the ball is placed before each play begins. The offensive team must start behind this line, while the defensive team lines up on or near it to prepare for the play. When the play starts, the offensive team tries to advance the ball past the line of scrimmage to gain yardage. The defensive team aims to stop them by tackling the player with the ball or preventing forward progress. This concept is crucial in the game of American football.