This device is used to control a 2 port valve on a heating system, and is basically a motor in a box.
This example is over ten years old and works intermittently.
Internally the device consists of a motor to move the valve to the open position, a spring to return it to the closed position, and a microswitch which is activated when the valve is fully open.
In a heating system when heat is required, power is applied to the motor to open the valve. The microswitch is used to activate the boiler once the valve is fully open.
Further details on heating systems can be found in the central heating section.