In rugby union, a penalty try is awarded if the referee determines that a probable try would have been scored, without some infringement occurring by the defending team. Many times this will result in a Yellow Card being issued.
For example, if an attacker and defender are both trying to touch a ball down in the in-goal area, but the defender hits the ball directly out (which is illegal), the attacker would be awarded a penalty try.
To determine if a probable try would have been scored, the defender is removed from the scenario.