Nomsfud is right that all of being&time is needed to really get Dasien. But, we can still try to explain it in a fallen/ontic way using non-Heideggerian language (read: understandable).
H. thinks all of philosophy is fucking up by taking what Being is for granted. So, H. using the phenomenological method, wants to understand Being. There is Being and being. [B]eing (little b) is all the stuff existing in the world; objects. Being (big b) is the state/action of existing: X (object) IS Y(quality); e.g. the sky IS blue. Being (big b) wants to understand the 'is', or, the structure of the conditions of possibility for Being, or what allows us to exist with and experience the world.
Dasien, super roughly, is the structure of people. Objects in the world are not Dasien. H. spends a good amount of time on objects in the world because the world is what is fundamentally experienced by us and therefore is the clue to get at Dasien's structure. We can't get at our own structure directly because it is too close to us (think losing your glasses on your head). In Heideggerian language, we are in a fallen mode most of the time. The way to escape this and see ourselves (or realize where our glasses are) is, pretty much, when something is fucked up: breakdown. When you sprain your wrist all of a sudden your like, "fuck, I'm in a body and my wrist actually does a lot". When your picking up a hot date in a car and the engine overheard your like, " fuck, this is an instrumental object ready-at-hand cue existential crisis" or when you can't see you look for you're glasses. Often, objects in the environment are what fuck s up and give us clues to the structures of being.
The rest of division 1 in being&time is about the spatial structure of Dasien, that is, the way Dasien IS in the world. Division 2 is about the hermeneutically deeper level of the structure, Time and finitude, which makes the spatial structure possible.
Some people say Heidegger was a pretentious dickhead and so wrote like one. I've never met the guy so I don't know but his writing style does coincide with his philosophy, I find. Also, his later work, Origin of the Work of Art, is way more accessible, maybe because he was less of a dickhead when he got older.
My brain is a little fried from exam period, hope this all makes sense!