In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. ... When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext.
Each letter of plain text is replaced by a letter with some fixed number of positions down with alphabet , numbers or special characters. To decode the message you need to enter the secret key.
This can be used for high security reasons ( RAW , Military , Bussiness etc ) to transfer secret messages.