What it is

Before the snap one eligible offensive player may run sideways or backward behind the line of scrimmage. This is called motion, and the player is the man in motion.

In a game

Motion tells the quarterback about the defense. If a defender follows the man in motion across the formation, it is probably man coverage. If the defenders stay in place, it is probably a zone.

More detail

The player must be moving parallel to the line or away from it when the ball is snapped, never toward the line. Teams also use motion to create a better angle for a block or a pass, and to move a fast player into a favorable matchup.

Animated diagram

1. Before the snap, the slot receiver starts to run across the formation, parallel to the line of scrimmage.
2. A defender runs with him. That is a sign of man coverage. If nobody follows, it is probably zone.
3. The quarterback watches the defenders move, and he reads the defense before the snap.