Question of the Day: What is a salt?
A salt is used in cryptography to make decryption less efficient for attackers by adding another hashing layer on top of an encryption algorithm. When a passphrase is used to encrypt data, a salt can be additional data that gets concatonated to the passphrase or key. This means that the attacker’s dictionary now needs to contain many more entries, one for each possible salt value for each probable passphrase.
Salts are implemented as random bits. They are used as a second argument along with the passphrase in a function that is used to derive a decryption key.
For practical purposes, you can use salts as a second passphrase equivalent across services, such as for example when interfacing with a third party web service that intends to be synchronized. By agreeing upon a common salt or salt algorithm, such as making it time-based, you can support handshaking while retaining an extent of cryptographic security.
For more information, the following Googled links are just a tiny few of the resources that describe salt in the context of cryptography and some of them provide a good introduction to cryptographic principles: