There’s not a solid answer on exactly what it is, though given that I made the video I’m sure I don’t need to re-iterate what I think. A lot of people, myself included, think that it was a part of the Lands Between that got hidden away because of a few different things. The Suppressing Pillar inscription and Miyazaki’s own statements make it pretty direct, in my opinion, but there are various other smaller things. I’ve seen some people argue that it’s an afterlife of a sort, or that it’s entirely within Marika’s dreams, but I don’t personally find those explanations particularly convincing or satisfying. Whatever it is, it was at some point “obscured by the Erdtree”, and there is literally a colossal veil hanging over the entire sky to reinforce that