Answer:
A mark occurs when a football has been kicked at least 15 metres untouched by any other person except by the person catching it. Such a catch is referred to as "a mark". The benefit of marking/catching the ball is that the receiving player is then able to take a kick unmarked by any opposition player. In many cases in the modern game, however, the player elects to 'play-on', which means he continues to play as if no mark had been taken.