One of the simplest and most known encryption techniques. It’s a type of Substitution Cipher in which each letter in are replaced by a letter some fixed number of positions down the alphabet.
For example, with a left shift of 3, D would be replaced by A, E would become B, and so on.
This is often incorporated as part of more complex schemes, such as the Vigenère cipher, and still has modern application in the ROT13.