xkcd

For the people out of the loop: salt is a term used for a piece of data to add to a password before hashing it, so two people with the same password will appear to have different passwords in the DB. So my password will be stored as xyz in the DB while Joe's password will be ABC, although we both use "hunter2".

It prevents an attack using so called "rainbow tables", which is a dictionary of hashes and their corresponding password (so xyz -> hunter2).