An actuator in simple words means a motor for moving or controlling a mechanism or system. The sensors that are planted on the satellite sense the direction and alignment of the satellite with respect to a reference inertial frame. The satellite then makes use of its actuators to stay in the orbit by controlling its motion and moving accordingly. The actuators can as well rotate the body to which they are embedded. In simple words, the actuators are responsible for the movement of the system they are attached to.