Zero Knowledge Proofs - What Are They?

Zero knowledge proofs have existed since 1985. They've been used in various areas before blockchain. The most obvious one is like authentication systems that use these schemes. If you don't want to share your password at all, I mean one way would be to hash it on the client side and then send that hash to them. But sending hash to them for something that's as small as the password is actually doing quite a lot of information.