Salts

If two people choose the same password, storing a plain fingerprint of it means both entries look identical. An attacker who cracks one has cracked both, and can see at a glance which accounts share a password.

A salt is a small piece of random data added to each password before it is fingerprinted, different for every account. The same password now produces completely different stored values. It does not make any individual password harder to guess. What it stops is one piece of work being reused across millions of accounts at once, which is how the largest breaches get cracked so quickly.