Icing is when a player shoots the puck from their side of the center red line all the way down to the other end of the ice past the goal line without anyone touching it. When this happens, the referee blows the whistle and brings the puck all the way back for a faceoff in the defensive zone of the team that iced it.
The rule exists to prevent teams from just dumping the puck down the ice every time they are under pressure. It forces teams to actually move the puck up the ice with passes or skating.
There are a few exceptions. Icing is not called if the team that shoots the puck is shorthanded. It is also not called if the referee thinks a player on the other team could have gotten to the puck first but chose not to. The team that ices the puck is not allowed to change their players before the faceoff, which makes icing even more of a punishment because tired players have to stay on the ice.
