Passwords / Explainer
A hash is a one-way fingerprint. The same password makes the same fingerprint when you use the same recipe, but the fingerprint is not the password.
pbkdf2_sha256$210000$randomSalt$derivedHash
This page's generator stores the algorithm name, the number of repeats, the salt, and the final hash. Those parts are enough to test a password later, but not enough to read the original password.
The password you type is changed into bytes, because hash functions work on bytes instead of letters on the screen.
A salt is random extra data saved beside the hash. It makes two people with the same password get different stored hashes.
PBKDF2 runs SHA-256 again and again. More iterations make guessing slower for attackers and still quick enough for real users.
The site saves the recipe and result. To check a login, it repeats the same recipe with the typed password and compares the new result.
SHA-256 mixes the input so strongly that a tiny change makes a totally different output. There is no undo button or hidden password inside the hash. The practical attack is guessing passwords and checking each guess, so salts and iterations make guessing much harder.