Tripping is a minor penalty in hockey - more specifically, in ice hockey. It's pretty much what you'd expect: if one player trips another, usually this is done with the stick, the offending player is whistled and sent to the penalty box for two minutes. During this period the penalized player's team is shorthanded and the other team has a power play. See those nodes for more information.
Sometimes, though, a penalty shot is awarded to the player tripped. If the tripped up player is on a breakaway - meaning he's going in all alone on the opposing team's goaltender and has a great scoring opportunity - it seems more fair and just to let that player have a penalty shot, or redo the scoring opportunity, essentially, that he was denied by the offense. Whether the penatly shot is successful or not, sometimes the offended team still gets a power play.