Disclosure as the happening of being takes place on three distinct levels that run from the original to the derivative: world-disclosure, pre-predicative disclosure, and predicative disclosure. The most original occurrence of disclosure is world-disclosure, the very opening up of the field of significance in conjunction with Dasein's becoming absent. In turn, world-disclosure is what allows entities to be meaningful present and to be known and used -- first of all, pre-predicatively -- within the various worlds of human concern. Finally, world-disclosure and the resultant pre-predicative disclosedness of entities, taken together, make possible the predicative disclosure of entities in synthetic judgments and declarative sentences of the type S=P. Properly speaking, the term "truth," taken as the correspondence between judgments and states of affairs, pertains only to this third level of disclosure where reason, logic, and science operate. Heidegger argues that the "essence of truth" -- i.e., that which makes predicative truth possible -- is world-disclosure insofar as it issues in the disclosed entities against which predicative judgments must measure themselves if they are to be true.
Heidegger calls the basic sense of disclosure (world-disclosure) "language," by which he does not primarily mean spoken or written discourse and the rules governing it. For Heidegger "language" means logos such as he thinks Heraclitus understood the term: the original "gathering" of entities into their meaningful presence so as to disclose them as what and how they are. This disclosive gathering happens only insofar as Dasein is "gathered" into its own mortality. "Language" in this original sense is what makes possible language/logos in the usual sense -- human discourse as the activity of synthesizing and differentiating entities and their possible meanings.
Heidegger argues that the disclosure of being as world-disclosure is intrinsically hidden, and he calls this hiddenness the "mystery" of being. The point can be quite mystifying until one realizes that Heidegger takes disclosure to be a unique kind of movement.
In Heidegger's interpretation, classical philosophy, and particularly Aristotle, understood movement not just as change but rather as the very being of entities that are undergoing change. Taken in this broad sense, movement refers to an entity's anticipation of something absent, such that what is absent-but-anticipated determines the entity's present being. For example, if you are studying for a university degree, that still-absent degree, as your anticipated goal, determines your current status as being-a-student. Your current being is to be moving towards the degree.