The Ancient Greek atomists believed that the essence of things, or at least, the essence of matter, could be found within matter. If you cut a piece of chalk into small enough pieces you would eventually reach a piece that could not be cut up any more, and that would be the essence of chalk, it’s fundamental building block. Modern day atomists are materialist scientists who believe that the only really real things are atoms, or at least the subatomic particles that make up an atom. Everything can be explained by getting back to the (sub)atomic level; there is nothing other than the behavior of atoms.
Heidegger is saying that in fact this is not true. It might be possible to cut something up till you can’t cut it anymore, but what you have not reached by that sort of ‘going inward’ is not, for Heidegger, the essence. You get his point when you acknowledge that the atoms that comprise chalk have no chalklike qualities at all. The chalkness of chalk lies way above the level of atoms, in their overall being together (standing consistently together in formed content).
Heidegger gets poetic about this by saying that as you cut something up, far from its essence getting revealed, it is almost as if a thing’s essence hides itself, fleeing from all this cutting up. That is why, Heidegger says, when you cut something up all you end up with is more inert stuff, as the life that holds that stuff together is off hiding somewhere.
Heidegger generalizes this point by arguing that the essence of ‘earth,’ his name for, more or less, the physical materialness of things, is withdrawal. Earthness shows up as a resistance to being shown (in the world), to appearing and being evident. Earthness is more like why aspects of things are difficult to explain, disappearing when you want to understand them. Heidegger considers this to be the way there is an aspect to all things that seems to be held back. The earth protects, harboring things from full disclosure, retaining something of the essence of things so that they are not all used up when appearing in the world.