"Ontological inquiry is indeed more primordial, as over against the ontical inquiry of the positive sciences."
– Martin Heidegger, Being and Time, §3.
Heidegger is an ontologist. To define or translate "Being" as "you" or "an entity" is sloppy and incomplete. Heidegger is not talking about the person, the literal being or entity. Rather, he's talking about the ontological difference between Being and being(s). The capitalization is significant there. His entire point is that there is a crucial distinction between a "being" (lowercase, referring to the entity) and "Being" (uppercase, referring to on ontological status).
This is his project of fundamental ontology, a reinterpretation of phenomenology that arose out of Heidegger's collaboration with Edmund Husserl (cf. "Logical Investigations"). In order to interrogate fundamental ontologies, he had to develop new language and/or use existing language in a naturally confusing way. On the very first page of Being and Time, Heidegger states:
"Our aim in the following treatise is to work out the question of the sense of being and to do so concretely."
Traditional concepts could not be used to articulate the argument that a phenomenon is the essence of a thing, because this distinction had not been previously made. For Heidegger, "Being" implies not only "subject" and "object", but also "body", "spirit", "consciousness", and "reality". Each of these concepts alone were inadequate and insufficient for his inquiry.
"'Being' is not something like a being. [… Rather, 'Being' is] what determines beings as beings, that in terms of which beings are already understood."
[ ... ]
"The question of Being aims…at ascertaining the a priori conditions not only for the possibility of the sciences which examine beings as beings of such and such a type, and, in doing so, already operate with an understanding of Being, but also for the possibility of those ontologies themselves which are prior to the ontical sciences and which provide their foundations. Basically, all ontology, no matter how rich and firmly compacted a system of categories it has at its disposal, remains blind and perverted from its ownmost aim, if it has not first adequately clarified the meaning of Being, and conceived this clarification as its fundamental task."
– Martin Heidegger, Being and Time, §3.
In some ways, "Being" can be understood—especially by the layman— as "essence", but that is problematic, too, since Heidegger is problematizing earlier views of "essence", which had situated "essence" as being located and observable in "phenomenon". Husserl had introduced a distinction between "phenomenon" and "occurrence"—a difference between representation in the world "as it is reflected in consciousness", and the essence of a thing "as it is in itself". Heidegger brought ontology into phenomenology, broadening the focus of Being beyond mere "consciousness".
This is why "Being" is primary. If one is unwilling to address things directly (zur Sache selbst), then no meaningful intellectual inquiry is possible. "Being" is present in a multiplicity of forms—me, other people, and other objects—all of which have their own subjective essences. "Being" is what makes beings (entities) intelligible as beings. It is itself something special, not just another "being" among beings, which is the mistake that Heidegger accuses Western philosophy of having made since Plato.
As always, the Stanford Encyclopedia of Philosophy has an excellent treatment of this topic, specifically in Section 1 of the entry on Martin Heidegger's Being and Time, but the entire article is worth a read. It moves onto explain the related concepts of "Dasein" (the being for whom Being is not a "what" but a "who"), "being-in-itself", "being-in-the-world", and "being-with".
Also, you really should read Being and Time. It's dense and certainly confusing, but at least it contains almost everything you need to understand Heidegger's argument within itself. It's one of those rare cases where you don't need a library's worth of background knowledge to understand the argument.