This program takes a text file and encrypts it by incrementing the ASCII values of each character in the file by +1. To decrypt a file, all the ASCII values of the characters in the encrypted file are decremented by 1.
Obviously this is not a method to do any serious encryption with, and you can reverse the functions (tell it to decrypt first and it will be 'encrypted' and encrypt the file to 'decrypt' it) to get the same result.