The plus/minus (+/-) stat is used to determine the goal differential when a certain player is on the ice. If said player's team scores an even-strength or shorthanded goal while he is on the ice, he would be a +1. If that same player's team is scored against while he is on the ice, he would be a -1. The plus/minus is never affected when either team scores a power play goal. So, if a player enters a game with a rating of +3 and is scored against twice, he is then a +1. If he enters with a +3 and his team scores even-strength twice, he is a +5.
The plus/minus stat is usually used to determine the effectiveness of a defender, though the stat is available for most hockey players.