Five on three (also called a two-man advantage) is a term used when one team has had two players sent to the penalty box. This leaves the opponent with five skaters (i.e., not including the goaltender) to the penalized team's three. The team with the advantage has a very good chance of scoring during these periods. Likewise, the team with the disadvantage will find it almost impossible to generate any scoring opportunities.
The team with the three players — nicknamed the iron three — usually forms a triangle in their zone with two defenders back near the goalie and one forward who goes around the slot and the points. They try to get their sticks and bodies into passing and shooting lanes and try to gain control of the pucks so they can shoot it down the ice and kill time off the power play and so they can go to bench and have teammates go onto the ice to fill in for them. Because of the lack of players, a team who has only three men allowed on the ice will almost never attempt a short handed scoring opportunity, even if a breakaway opportunity presented itself, mainly because if it were to miss, the shooter would be well out of position, leaving five offensive attackers to face only two defenders, which in turn leaves the goaltender in a very difficult position. Instead, the team will use the most conservative, defensive strategy available to them and attempt to wait out the penalty until one of the penalized players returns to the ice.
A common way to combat the triangle is to get one of the five players inside it. If they can get inside and get the player the puck, the three man squad is put in a precarious position. The five mainly try to cycle the puck around amongst each other trying to draw one of the penalty killers to follow them or get him out position so they can gain a good shot on net or set up a one timer.