The door handle is used to open the door, and as a way to release the door latch. It mates with a toothed wheel at the side panel of the door, called a rotor. When the door is closed, one of the teeth contacts the striker and causes the rotor to turn one tooth. If the door is completely closed, the rotor engages with the striker. The door can't open because the rotor is locked. When you unlock the door, the rotor can rotate and the door opens.