When is a door not a door?
A door is not a door when it is ajar, or open. When a door is open, it is no longer fulfilling its function as a barrier, and is instead allowing access through an opening.
how can a door be a jar?
If you are asking me how a door can be ajar then a door is ajar when it is slightly open, leaving an air gap. 

Here are some examples:
The door was ajar.
My email is open and Twitter ajar for your musings.
Turn off the oven and put the cheesecake back in with the door ajar.
The lid had come slightly ajar; the bird was straining at the gap.