Tall Paul is right as far as he goes. I assume you are referring to the little triangular plate mounted on sort of a pylon ahead of and below the aileron. The little triangular surface provides a torque opposite to the resistance of the aileron in order to reduce the control force required to move the aileron out of the faired position. The weight moves the cg of the control surface to a location at or ahead of the hinge line to avoid a tendency to flutter. Many production aircraft have the counterbalance weight concealed within the leading edge of the aileron where it isn’t obvious. But all have some way to keep the control surface balanced near the hinge line to avoid flutter.