Semantic Foundry explains what makes stuff possible
[I]t’s important to realize at the outset that for Heidegger, [B]eing is neither a substance nor a process. Being, in early Heidegger at least, is “that on the basis of which beings are already understood.” One might say that the understanding of being is the style of life manifest in the way everyday practices are coordinated. A culture’s understanding of being allows people and things to show up as something — people show up as heroes in Greece and as Saints in the Middle Ages, for example, and things for the Homeric Greeks were flashing up to be admired, while for Christians they were creatures to be mastered and interpreted.
Put generally, the shared practices into which we are socialized provide a background understanding of what counts as things, what counts as human beings and what it makes sense to do, on the basis of which we can direct our actions towards particular things and people. Thus the understanding of being creates what Heidegger calls a clearing (Lichtung). Heidegger calls the unnoticed way that the clearing both limits and opens up what can show up and what can be done, its “unobtrusive governance.” (Waltens)
For Heidegger the history of being in the West has been the history of misunderstandings of the clearing. From Plato on, philosophers have sensed that something beyond ordinary beings was responsible for their existence as anything, but since the clearing always stays in the background — or, as Heidegger puts it, withdraws into shadows — philosophers have replaced it with a highest being that is the ground of [beings] and the source of their intelligibility. For Plato the highest being was the good, for Aristotle the Unmoveble Mover, for the Christians the creator God, and after the Enlightenment it was ||MAN|| himself. Heidegger calls all these attempts to replace the clearing with a “beingest being”, onto-theology or metaphysics.