"What does Heidegger mean by "being"? The single most important point to grasp at the outset is that being is not itself something that exists: it is neither one entity among others, nor the totality of entities (das Seiende), nor a property of entities. The difference between being and entities is what Heidegger, soon after writing Being and Time, calls the "ontological difference" (GA24: 22). Since his question concerns being, and since being is not an entity, Heidegger is not primarily interested in the central question of traditional ontology: what is there? Are there forms and universals or only particulars? Does God exist? Is there such a thing as substance or are there only properties? Is the mind physical? Do we have free will? Are we and everything else ultimately will to power? These are metaphysical questions; they are questions about entities, not about being. They are, in Heidegger’s jargon, "ontic" as opposed to "ontological." Moreover, by fixing our attention exclusively on entities, Heidegger believes, such questions tend to eclipse and obscure the question of being. The difference is not just one of generality, for Heidegger also distinguishes the question of being from what he later calls "the fundamental question of metaphysics," namely, why is there something rather than nothing? (GA40: ch. 1) The question of being is not about what there is or even why there is anything, but rather what it is for what there is - whatever it is, and for whatever reason there is any of it - to be.
What then is "being"? The closest Heidegger comes to a definition is to say that being is that in virtue of which entities are entities; it is what makes (in a noncausal sense of "makes") entities entities. This  should not mislead us into supposing that being is a property of entities. Aristotle and the medieval scholastics knew that "being" does not name a peculiar feature of a kind of entity, or even entities as a whole, since a contrast class is by definition out of the question. What would "entities" lacking being be? They would not be entities at all. As Kant observed in his refutation of the ontological argument for the existence of God, being may be a "logical" (and linguistic) predicate, but it is not a "real predicate" or property.  On the surface, the sentence "Dogs exist" looks grammatically the same as "Dogs bark," but the surface grammar is misleading. We know what non-barking dogs are, but what are nonexistent dogs? What would entities be without the putative property of existence? Nothing. And what could actually have such a property? Only entities. Yet the entity-ness of entities is just what possession of the property was supposed to explain.