That's a really broad question of sorts. But Gewörfenheit which you render as thrownness refers to Da-sein (for our purposes you can just think of this as "human beings" ) state of already being the world. In other words, we don't start out at the view from nowhere.
There are some corollaries for Heidegger that follow from this:
- Da-sein is always in motion propelled as it were.
- Da-sein is in motion towards our its death (this is the ultimate possibility for Da-sein)
- Da-sein is caught up in time.
Da-sein has no choice in any of this and just needs to figure out what it will do given this. For Heidegger, the answer is to be authentic about possibility #2 and understand our state in this way, accepting the limits of our existence...