i can tell you more if you want, i am not a stranger of philosophy, even if heidegger is not my favorite.
p.s the Da-Sein(being here/there) is the human being, not an abstract concept of his philosophy.

ok.being and time, by Martin Heidegger. it is really a hard book to read an not being in college. but this says a lot.
i will try to explain, in very short terms the idea of this book.
in the first place, you really must read ancient greek philosophy to understand. to problem is that we have forgotten the essential question about the Being(try to read about Parmenide-the One and Mulptiple dillema, Plato-the dialogues and Aristotle-metaphysics).but he is often regarded not as a phenomenologist(he was the disciple of Husserl, father of phenomenology)but as an existentialist(as Kierkegaard-wich I recommned if you are interested abou it).now, this is because we are thrown(throwness-Geworfenheit)in this world without knowing why and by whom.this create a state of anguish wich is an existentialist one.in the end of the book he will talk about time, the authentic time, in wich we project ourselves and our possibilities. but we project ourselves toward our death. this is were the book stops.Heidegger had in mind a second part, but under the meetings with Karl Jaspers(renowned psychanalist) he gaved up. this is the verrrrrrrrrry short version, vulgata heideggeriana.;)