Please, feel free to update and improve it.
Don't use this app to ACTUALLY crypt some vulnerable information of yours.
It is a type of substitution cipher in which each letter in the plaintext is 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.
It's a not-so-efficient algorithm you can use to prank your not-so-programming friend :D