A plane is a separate plane of existence. Up until lord a lowmort or hero existed on the Midgaardia plane (except the locker rental room). However, once they ascent to lord they are transferred to Thorngate and learn to transfer between planes with Planeshift and a key word representing that plane or a special artifact called the Planar Anchor which can be placed in certain locations. Each plane has one room you are shifted to. In the case of Midgaardia that is the room Zin is in at the Trials Of Zin.