Point in a
hockey game where a set of players, or
line, switches out with another.
Ice hockey is such a fast-paced sport, and has so few stoppages, that, unlike most other sports, multiple sets of players are needed, and they have to be able to enter or exit play while it is still continuing. Usually, a set of forwards (left wing, center, and right wing) works together, as well as a set of defensemen (left and right). These sets are called lines.*
To change a forward line or entire team of skaters on the fly, the team usually waits to gain possession, then one of the exiting forwards will take the puck, advance just past the red line (to avoid an icing call) and shoot the puck into the opposition's end of the ice. While the opposing goaltender and defensemen are recovering the puck, the exiting players leave the ice and the new players enter, usually hopping directly over the boards. The new forwards head up the ice to forecheck, while the new defensive pairing (if any) hangs back and awaits the potential rush. This strategy is called "dump and change."
A defensive-only line change is simpler -- as long as the puck is in the other team's offensive zone and their team has possession, defensemen can safely leave the ice just about anytime. They usually go on an offensive rush or the aforementioned "dump and change," though.
* Sometimes forward lines are paired with defensive lines, sometimes not. It depends on the
coach's philosophy, as well as the players he has to work with.