Defensive offside is a penalty in Football that is called on the defensive team when a defensive player crosses the line of scrimmage or neutral zone prior to or simultaneously as the snap. So long as the defensive player does not make contact with an offensive player, the ball is not snapped, and an offensive player doesn't false start due to the offside, the player may return to their side of the line of scrimmage without penalty. If any of the listed action do occur, such as the ball being snapped while the player is across the line of scrimmage, an offside penalty will be enforced, which is a 5 yard loss for the defense.
If the offense is quick enough to snap the ball while a defensive player is offside, they will get a 'free play', wherein they may attempt a deep pass or a risky throw, but they do not have to worry because they can simply accept the penalty and replay the down if the play doesn't work out.