Looking at a heap of sand or, if you like, a sandcastle, both are devoid of the substance sand. The abstract “heap-ness” or the abstract “sandcastle-ness” depend upon sand thus they are nothing in themselves. The heap and the sandcastle are dependent originations, in other words.
Everything that is dependent is unessential or the same, lacks true essence. Dependent existence is only through essence which is independent of the contingent nature of such existence.
Turning to our psychophysical body, the five skandhas or aggregates, consisting of material shape, feeling, perception, habitual tendencies and consciousness lack essence thus being also empty. They depend on the essential just like a heap-ness needs sand.
Likewise in the world of human thought-forms or concepts they, too, lack essence. No thought-form is the Buddha Mind or pure Mind. No thought-form is nirvana and so on. We can say this of what we imagine, for example, an imagined Buddha is not the Buddha and so on. As Buddhist seekers our goal is to see the very substance or our thought-forms which is pure Mind. Seeing this, suddenly we understand what the old Buddha was talking about; we see eye to eye with the Zen masters, too.