A salt is a random addition to a password to make the hashed password less susceptible to a lookup table attack
In cryptography, a salt consists of random bits, creating one of the inputs to a one-way hashing function. The other input is usually a password or passphrase. This salt can be stored with the hashed password.
The benefit provided by using a salted password is making a lookup table assisted dictionary attack against the stored values impractical, provided the salt is large enough.