A rainbow table is a precomputed table used in password cracking to reverse the process of hashing. Hashing is a one-way function that takes an input (such as a password) and transforms it into a fixed-size string of characters. It is commonly used to store passwords securely by storing only the hashed version of the password rather than the actual password itself.

Rainbow tables are used to speed up the process of password cracking by storing a large number of precomputed hashes and their corresponding plaintext values. These tables allow an attacker to quickly look up a hash and find the original plaintext value without having to compute the hash for every possible input.