1. Das Sein = das "ist"
a. The guiding question (Leitfrage) of Western metaphysics is the question about the being of entities, das Sein des Seienden. But the being of entities is the "is" of entities -- in Heidegger's words, "das ist, d.h. das Sein" or as William J. Richardson puts it, "the Is of what-is." "Being" shows up only as what, that, and how an entity is (das Was-sein, Wie-sein, Daß-sein des Seienden). It is also called the "is-ness" of what-is, die Seiendheit des Seienden (GA 9, 260.6-7).
b. For Heidegger, however, unlike the classical tradition of metaphysics, Sein or ist is not an ontic characteristic of entities in themselves and does not occur on its own-apart from human beings. To be sure, Sein/ist is always the "being/is" of an entity. But in Heidegger's phenomenological view, such "being" appears only within a human enactment of legein (= hermeneuein, Seinsverständnis), a synthetic-differential act of taking-an-entity-as (ti kata tinos legein = etwas auf etwas entwerfen). The orginal manifestation of Sein or ist occurs as the "as" of anact of taking-something-as. In theorectical objectivity we take an entity as this-or-that; in practical engagement we take the tool as-suitable-for a task. Thereby, whether thematically or unthematically, we understand that the entity is this-or-that, and that the tool is for a certain purpose. Only in this way do we know an entity's "being".
c. Heidegger, both early and late, holds that "being" is phenomenological. It occurs only in a synthetic-differential act of taking-as; only as the what-ness, how-ness, and that-ness of the entities we encounter; and only as the sense or meaningfulness of those entities. To say "This entity is ..." is to say that this entity "makes-sense-as." Das Sein = das Anwesen-als = die Bedeutsamkeit [sc., des Seienden].