The replacement of the forward and/or defensive lines on the ice with other lines whose players were sitting on the bench. Line changes happen around once every minute while play is going on, and they happen almost every time there is a stoppage in play. Making a line change while play is in progress is called changing "on the fly", and any player coming off the bench during a change on the fly must wait for a teammate to leave the ice surface before they can step out onto the ice. Head coaches and/or assistant coaches decide which lines are sent out onto the ice.
Strategy is involved with line changes, as coaches like to match certain players against certain opposing players on the ice. For example, a coach may choose to send out a line comprised of defensive specialists each time the best offensive players of the opposing team step out onto the ice. Coaches can choose to make changes on the fly in attempting to get a favorable match-up for them, which forces the opposing coaches to adjust to the on-the-fly changes.
Teams who ice the puck (or who are called for icing) are not permitted to perform a line change during the stoppage of play resulting from the icing. In every NHL game, the home team has the benefit of having the last change, meaning that they can choose which players to send out onto the ice after the opposing team sends out theirs, allowing the home team to get favorable match-ups.