"Illegal motion" is the name given to a five-yard penalty called against the offense when an offensive player is in motion towards the line of scrimmage at the time of the snap.
Offenses often send players in motion prior to the snap because it allows the quarterback to see what type of scheme the defense is using based on how they react. Players also go in motion if the quarterback changes the play at the line of scrimmage. When players go in motion they must come to a complete stop and pause before the snap, otherwise illegal motion is called. This prevents wide receivers from sprinting towards the line of scrimmage at the snap to get a running start for the play.