Every door has a frame. This is a wooden area. The door is attached, using something that we call hinges, to one side of the door frame. If you were to draw an imaginary line from one side of the door-frame to the other then a man standing on this line would be standing 'in the doorway.' We use the expression 'at the door,' to mean that somebody is standing outside a closed door and has just rung a bell or knocked on the door to ask to be let in.