What is a one-way hash function?

A one-way hash is easy to compute forward but computationally infeasible to invert.

f("M1s!ecurePW") = "Wre/JFp9qIlxKldMuuuD3cnktDfWR"  (cheap)
f_reverse("Wre/JFp9qIlxKldMuuuD3cnktDfWR") = ???    (very expensive)

The hash function must be cryptographically secure:

• MD5, SHA-1: Broken - don't use
• SHA-256, SHA-3: Strong for general hashing
• bcrypt, Argon2: Recommended for passwords

Go deeper:

• bcrypt (Wikipedia) — why a slow, adaptive hash beats a fast one like SHA-256 for passwords: the cost factor can be raised as hardware speeds up.