Salt
Random data that is used as an additional input to a one-way function that hashes data
A salt is simply added to make a password hash output unique even for users adopting common passwords. Its purpose is to make pre-computation based attacks unhelpful. If your password is stored with a unique salt then any pre-computed password-hash table targeting unsalted password hashes or targeting an account with a different salt will not aid in cracking your account's password. A long randomly generated salt (using /dev/urandom) is expected to be globally unique. Thus salts can be used to make pre-computation attacks totally ineffective.
The simplest way to combine the salt and the password is to simply concatenate them, i.e. the stored hash value is Hash(salt||password). The common password password1 now magically becomes, e.g., 6$dK,3gCA%Jpassword1 which is unlikely to be found in a password cracker's table.