He lay in a bright mist, though it was not like mist he had ever experienced before. His surroundings were not hidden by cloudy vapour; rather the cloudy vapour had not yet formed into surroundings. The floor on which he lay seemed to be white, neither warm nor cold, but simply there, a flat, blank something on which to be.
Harry Potter and the Deathly Hallows
How should we understand "on which to be" here? I just can't figure it out.