Hashing
A one-way function that gives the same input the same output every time — but you can't go backwards.
Run "hello" through SHA-256 and you get 2cf24d…. Run "hello" again and you get the exact same string. But you can't take 2cf24d… and reverse-engineer "hello".
Used for: storing passwords (you compare hashes, not text), file signatures (verify a file hasn't been changed), blockchain. Password hashing should use bcrypt, argon2 or scrypt — not SHA-256, which is too fast.