What are "property descriptors"?

[pejato (Peter Tolsma)]

Basically the title.. When reading through the stdlib implementation of keypaths, I see quite a few references to these. There's a comment in SIL.cpp that says

// The storage needs a descriptor if it sits at a module's ABI boundary,
// meaning it has public linkage.

So, I guess property descriptors provide some description of properties that are exposed to linking?

Any other info about the implementation details of key paths would be appreciated. I'm interested