Best way I can explain it is with an example: if you're religious, think of Heaven and Hell. They dont exist in this plane of reality, they exist in their own plane of reality. They are two different dimensions, and our world we live in is also a different dimension from those two. Now imagine Heaven, Hell and our universe aren't the only dimensions. What if there are other dimensions? What if the beings from those dimensions could open a portal and travel to our dimension? Is it even possible? Do other dimensions apart from Heaven, Hell and our dimension even exist? I wish I could answer those questions, but we'll probably never know.