The door lock actuator is a small electric motor and gear assembly integrated into the door latch mechanism. When you press the key fob or the interior lock button, the actuator physically moves the latch from the “locked” to the “unlocked” position. In modern vehicles, these are specific to each door (e.g., front right, rear left). A failing actuator is usually identified by a whining noise when locking, or a specific door that refuses to lock/unlock electronically while the others work perfectly, forcing the use of the manual key.