I love questions like these! Something everybody thinks they know the answer to, but the more one thinks about it, the less clear it is.
Of course, the first thing that pops into my mind is that a hole is an 'absence' of something. But what does that mean?
Well if you have a muffin, and you remove the middle, you have a doughnut (sort of) . That is what you might call 'an absence of muffin.' And hence a hole.
But what is the part of the muffin you removed...(if you haven't eaten it yet)? It is not an absence of anything. It is a something. A mini-muffin perhaps. In Canada they call them "Tim Bits" at Tim Hortons. They are in fact doughnut 'holes'. But are they really holes? The hole is back with the original doughut they were extracted from.
So you see how quickly a simple question can blossom into a fascinating and tasty conundrum?
A great question to ponder and of course I have only scratched the surface. I'm sure there is a mathematical type out there who will tell us what a hole is in terms of an equation. Or a physicist who tell us a hole is just a figment of the human mind as in fact nothing in space is truly 'empty'...being filled with 'dark matter' and so on.
Holy Smokes! What a question!