A valve actuator is a control/force mechanism attached to a valve which will move the valve to the desired position on demand.
Usually a command from a control system would initiate the actuator to propel the valve into a fully-open or fully-closed position, or a more open or a more closed position.
The power source may be pneumatic, electrical, or hydraulic.
Depending on the application these may be very simple or exceptionally complex devices. In either case, the valve actuator must "fit" with operational requirements and with the valve system in use.