It is a shimmering white place with huge columns that seem to go far into the sky. From that place, other worlds can be accessed. It is entirely possible that the Hallway is another dimension, since it is entered through a Black Gate (which, according to the Xorinite Wisp, enables interdimensional travel). The Avatar entered the Hallway at the end of Ultima VIII with the help of the Obelisk, and went through it to return to Britannia. The Hallway could be a dimensional crossroads from which a person could enter any world or dimension desired.