On the tailend of an airplane, hinged control surfaces on the horizontal stabilizer -- the mini-wing at the tail of the plane -- are the elevators. The position of the elevator controls whether the nose of the airplane is pointing up or down and thus moving up or down.
The nose of the plane moves in the direction of the elevators. Point the elevator up and the nose goes up and the airplane climbs. Move the elevator so it is pointing down and the nose goes down and the airplane descends.
Not all RC airplanes have elevators. Those type of planes rely on other means such as thrust (power to the motors/propellers) to ascend and descend.
