This is a playing-around example of the One-time pad algorithm. If done correctly, and I am not saying this is done correctly, it is impossible to crack.
Type some sort of key in the Key Text field. This can be any text given it is as it is as long (or longer) than the Plain Text - this is the "private key" that would need to be shared between the two parties.
After you have the Key Text typed in, type in the Plain Text or Cipher Text and then click either Encrypt or Decrypt respectively.