The offside rule is a hard rule to understand when first watching the game of hockey. Basically, this rule stipulates that the puck has to enter the offensive zone (from the blue line to the net) before any player of the attacking team can enter that zone. If a player of the attacking team enters the offensive zone before the puck, all attacking players must be outside the offensive zone before they can re-enter it. An infraction will only be called if an attacking player touches the puck while in the offensive zone. A faceoff will then take place outside the zone, near the blue line.
This rule helps the defensive team greatly as all they need to do is shoot the puck out of their zone to force all the players from the attacking team to exit the zone in order to avoid a faceoff.