A powerplay in hockey is really quite simple. If a team commits a penalty, it is forced to play without one of its players for a duration, depending on how severe the penalty was. For a minor penalty, the players sits for two minutes, a double-minor penalty for four minutes and a major penalty for five minutes.
This gives an opportunity to the opposing team to play with a man-advantage and thus have one player that is not being defended.
In most cases, teams see this as an advantage to score a goal. The Chicago Blackhawks have not been so fortunate.