"Offsides" refers to a penalty when a defensive player crosses the line of scrimmage prior to the snap of the football. Offsides can also be called on the kicking team if a player crosses the line before the kicker makes contact with the ball.
Defensive players try to gain the advantage by rushing the offensive line as soon as possible, which causes the penalty. Offside calls result in a 5-yard penalty for the defense. If the defensive player is offsides prior to the ball being snapped, there is no penalty and they are allowed to go back to their side. If the ball is snapped and the defensive player does not make contact with the offensive team, the play is allowed to be completed.