It's the portion of the control surface which resides on the opposite side of the hinge. For example, a way to have a counter balance on the rudder is to make the add-on go up and over the top of the fin. When the main part of the rudder (behind the hinges) goes left, the counter-balance (ahead of the hinges) goes right. It eases the servo load by balancing out some of the force on the control surface.