What is door ajar on Lincoln town car signature series?

The "Door Ajar" warning on a Lincoln Town Car Signature Series (or any car, for that matter) means that at least one of the car's doors is not fully closed. The system detects this and illuminates a warning light (usually on the instrument panel) to remind you to close the door(s). This is a safety feature to prevent the car from being driven with a door open, which could be dangerous.