Ah, this is a pithy one!
Simply look at your own basic awareness, mind, noting that if you pursue it to the deepest level (which means spending a lot of very still, silent time) there is nothing there.
No color, no shape, no size, no attributes or qualities – just awareness. Sometimes referred to as “pure awareness.” Awareness that has no content. Essentially, we realize that awareness is simply the potential to be aware of some content. So the mind, in itself, without anything else, is nothing.
Pursuing this, eventually we see that the nature of everything is impermanence, emptiness or shunyata – not that it doesn’t exist, but simply that everything is empty of an independent, abiding nature. So it doesn’t exist in and of itself, it only exists in co-existence with everything else.
As I said, pithy. You might have guessed that this is the essential thing you must get before much else in the Buddhist meditation catalog really works for you…