Note: This page is taken from n.wikia.com.
(As written in the N in-game help)
These will stop weapons and drones. They open and close automatically -- just like in Star Trek!Normal doors are barriers that open automatically when the ninja is nearby. Their primary function is to keep a drone, a thwump, or a floorguard confined to an area or to prevent the ninja from being attacked by rockets, gauss turrets, or a drone's ranged attacks.
If the ninja moves into a space (the square area occupied by a tile) that is adjacent to the normal door, the door opens. While the door is open, enemies and projectiles can go through it. When the ninja moves away again, the door closes. Because a door opens any time the ninja comes near it, they rarely obstruct or hinder the ninja's movement significantly.