This was the first well known cipher, a substitution cipher, and was used by Julius Caesar around 58 B.C. Because of this it is now known as the Ceasar Cipher. It was used by military leaders for hundreds of years after Caesar.
Caesar shifted each letter in his military commands so they would appear meaningless should the enemy intercept them. The shifted message is known as the encoded or encrypted message. You have to know what the shift is in order to decrypt the message. Caesar's Cipher was not broken for 800 years, until a mathematician used frequency analysis to break the code.