Definition
Can be executed only by a player inside their own 25yd area or in the in-goal zone. The player must catch the ball from the direct kick and shout, "Mark" in a clear voice. If this is done successfully then a free kick is awarded. It is also a way of legally stopping play.
Videos containing the term 'Fair catch'
http://www.sportsdefinitions.com/rugby-union/Fair-catch.html