* Door ajar: This is the most common reason. Many cars won't start if a door (or trunk/hood) is open or improperly closed, as a safety precaution to prevent accidental roll-away. In this case, the car likely *will not* start until the door is properly closed.
* Other sensor issue: The light might indicate a problem with a door sensor, a malfunctioning door lock, or even a more serious electrical issue that's unrelated to the door itself. The car *might* start in some cases, or it *might not*.
* Security system issue: In some vehicles, the light could be related to the alarm system or immobilizer. This often results in the car not starting.
In short: Check your car's owner's manual to understand what the specific red light on your driver's side door means. If it's simply a "door ajar" warning, close the door firmly. If the problem persists, consult your owner's manual or a mechanic.
