A mantle is a move which requires the climber to pull themselves up above a hold (usually a ledge or often the top of the climb), then at the critical point, switch forces and begin push down on the ledge.  This force allows the climber to place their feet and hands on the same ledge, there by completing the move.  The move is similar to how a swimmer climbs out of a pool, although is obviously more controlled. 

Mantles are great fun and require a bit of practice to get used to, but once complete, you’ll be amazed at how comfortable you can feel with both your hands and feet on the same ledge.