What does "hairline threads" mean here?
"The screw through Cinder's ankle had rusted, the engraved cross marks worn to a mangled circle. Her knuckles ached from forcing the screwdriver into the joint as she struggled to loosen the screw one gritting twist after another. By the time it was extracted far enough for her to wrench free with her prosthetic steel hand, the hairline threads had been stripped clean."
I believe in the context of this passage the "hairline threads" have something to do with mechanics, but I can't understand what exactly the sentence means.

Question

Answer

'Threads' is a noun used for the raised part, circling part of the screw. The thread goes around the screw in a spiral pattern, and is what causes the screw to go in or out when it is turned.

'Hairline' is more commonly used to describe a crack in an object like a plate, that is very thin, almost invisible, and so might be mistaken for a hair resting on it's surface.

It is not a common combination. 'Hairline threads' simply means that the threads are very fine, perhaps delicate, perhaps thin.