Yeah, I've heard about this.
If the 4th, 5th, 6th, etc. dimensions actually exist, in forms other than space or time, I can see how this is plausible. It doesn't even fully make sense to me, but basically, we live in the 3rd dimension and can only see if 3 dimensions or less. For all we know, 4-d things can be all around us, but we can't see them, only the bits of them that penetrate into our dimension.
Correct me if I'm wrong, but that's what an hour of YouTubing a while back taught me.