If you consider the universe to be a well-defined volume, then there must be some boundary to it. And if there's a boundary, then you can ask the question "What's on the other side of the boundary?". Thinking of the universe as a 2D surface in a 3D space is a way to picture how it's possible to have a finite space with no well-defined boundary.
Well, it's not clear to me why we would assume there is a boundary. And I don't know what it means to think of a 3D area as a 2D surface anyway. And I don't think it's possible to have a finite space with no well-defined boundary.
Oh. Yes, now that you have mentioned it, I do recall "space" being used in math classes in a unique way. It never occured to me that that is how "space" was being used.
