What follows is a list of definitions of terms and clarifications of those definitions.
For Heidegger, Being is whatever transcends our reality/existence. Traditionally, philosophers have understood Being as a substance or a kind of stuff. Heidegger wants to avoid this substance ontology and instead is concerned with Being as something that transcends all beings, or all things in our reality.
Whereas Being is the most universal concept, the fact that it transcends our reality means that it is most obscure, so we cannot think of its universality in terms of a genus to a species, as philosophers have traditionally thought. Another prejudice of traditional ontology is to think that just because Being is not able to be defined in terms of beings (things in our reality), this eliminates the question of Being. But Heidegger thinks we just need a new kind of questioning. Although we lack clear knowledge about Being in spite of that fact that it is a self-evident concept insofar as it is part of everything, the reason why we have no clear knowledge of it is because its everydayness obscures it.
So rather than do traditional ontology, Heidegger wants to do fundamental ontology as the existential analysis of Dasein. Dasein, or people, are the kinds of beings who in their Being as a being are concerned with their own Being. In other words, part of our essence as physical beings is to be concerned with what transcends our own existence.
Dasein is also able to reveal truth. For Heidegger, truth is what we call it when something is revealed as it is itself. Truth as uncovering or revealing is contrasted with the traditional correspondence theory of truth, according to which sentences are true insofar as they correspond to reality. In other words, the traditional theory of truth says that statements are true because they accurately represent some fact about the world. In contrast, Heidegger thinks that truth is in the things themselves when they are revealed as they really are.
The title of the book, Being and Time, reflects the fact that Heidegger thinks that time is the basic meaningful structure for our existence. Dasein is its past and its past is always ahead of itself. To say that Dasein is its past means that Dasein is shaped by its history. To say that its past is always ahead of itself means that our future is also shaped by our history. These features of the meaningful structure of time are the result of Dasein's ecstatic temporality. Although the experience of time is as of a series of 'now' moments, the meaning of time is such that our past shapes our present and future. In addition, our present determines how we interpret our past and how we envision our future. Likewise, our envisioned future changes how we interpret our past and our present.
