Rainbow tables are large dictionaries of common words in the dictionary and names, as well as their permutations, that are accompanied by their resulting hash in the desired hash algorithm. This is used by attackers that which to access user accounts, as many passwords are typically weak and can be easily guessed. While this isn't to necessarily access the accounts through brute-force attempts, it is used after hashed passwords have been exposed to the public once a database has been compromised.