Any human being that exists in the world is Dasein (being there/here) as they are a meaning-generating conscious entity that is aware of their own existence. Dasein does not refer to the entity’s “what”, but rather it’s Being, and every human has their very own Dasein, thus Dasein has in each case “mineness”. Dasein is always in situation in the world, not in the sense that water is in a glass, but primarily by being along entities in the mode of their being ready-to-hand.
As Dasein is Being-in-the-world, our relation to the world is our relation to those entities that are ready-to-hand by which we are along; our worldhood of the world comes from seeing the entities as their aspect of use, we interepret the entities to understand the meaning of being. To understand this, Heidegger explains that things always present themselves in some background, where the background is my experience with these things, and are always in situation in the world.
It is by being In-der-welt-sein that our possibilities of authenticity and inauthenticity arise.
However, Dasein’s who-ness is always related other people, as it is always in the world along with other Daseins. Even when we are rejecting the others or avoiding them, our Dasein is still defined by the relationship (or lack thereof) that we have with them. The Others are not definite others, and are not represented quantitatively, but rather can be represented by any Other who proximally is there in everyday Being-with-one-another (p.35), the they (das Man).
Our relationship with Them is always one of care and concern, as we are always preoccupied with how we are perceived by Them. This creates distantiality, and thus takes me out