HKDF
A key derivation function (RFC 5869) that extracts entropy from input key material and expands it into multiple independent keys.
HKDF has two steps. Extract runs HMAC over the input keying material with a salt to produce a uniformly random pseudorandom key. Expand runs HMAC again with an info string to produce as many output bytes as needed. Different info values give cryptographically independent keys from one secret.
It is used to turn a Diffie-Hellman shared secret into session keys (the TLS 1.3 key schedule is built on it), to derive per-purpose or per-object keys from a master key, and to derive fresh AEAD keys per file so nonce limits never bite.
HKDF is for high-entropy inputs. It is not a password hash: it is fast by design, so passing a password through it gives an attacker cheap guesses. Use Argon2id for passwords.