Final answer:
Rainbow tables are lookup tables used to speed up the process of password guessing by providing precomputed hash values for a large number of possible passwords.
Step-by-step explanation:
Rainbow tables are lookup tables used to speed up the process of password guessing. These tables contain precomputed hash values for a large number of possible passwords. When a password hash needs to be cracked, the rainbow table is consulted to find a match, providing a shortcut to finding the original password without having to go through the process of hashing every possible password.