The actuator is a motor which is part of an automatic transmission system and its job is to push the clutch release arm, removing drive from the engine to the gearbox. In a manual vehicle this role would usually be performed by the driver pressing their foot down on the clutch pedal.
The actuator is self-adjusting which means that when the clutch cover and pressure plate wear down the actuator is capable of compensating for their wear. This adjusting mechanism is sensitive and, once an actuator is removed it will be destroyed, so if the actuator shows signs of failure it is not possible to repair it and it should be removed. When the actuator is replaced it is essential to also replace the clutch cover, plate and bearing alongside it to ensure correct operation.
Symptoms of a actuator being faulty can include frequent stalling, an inability to engage drive from start-up and depending on the make and model of the vehicle, or a warning light appearing on the dashboard alerting you to a fault. On inspection of the actuator you may find that it appears hot, which can indicate a jammed part.