Heidegger's term for the unchosen conditions into which you are born: ancestors, tongue, customs, land. Heritage is the bedrock of identity.
Thrownness refers to the fact that you are thrown into the world, descended from particular ancestors at a particular time and place, given your mother tongue, moral intuitions, customs, and genetics. Your heritage is fundamentally what makes you you, and it is entirely unchosen.
The liberal rejects heritage, replacing organic identity with propositional identity. For the liberal, you are what you believe. The folkish worldview sees propositional identity for what it is: weak. An identity you can be argued into, you can just as easily be argued out of. It binds only so long as the individual feels like being bound.
The original meaning of "freedom" was not unboundedness but belonging to a common stock. To be free was to belong to something; in fact, to be unable to be rid of it. Real authenticity just is thrownness.