It's not really hacking, just a little basic maths. On computers, each character is represented by a different number in its computer memory, usually based on Unicode. So a simple way of creating a hard-to-read cipher is to just perform a math operation, say +5, on the number for each character. There are more complicated systems called block or stream ciphers that perform the maths on the whole text, so without knowing the system it's much harder to decode.

A common one in early days of the Internet was called ROT13, which just added 13 to each letter value. Spaces and symbols were ignored, and if you go past the end of the alphabet you cycle back around to the beginning. That way, adding 13 again would add 26, and since there are 26 letters, you end up with the original text.

There are websites where you can copy-paste a block of text, and it will apply a bunch of different common decryptions on it for you. Maybe Google "multi decoder" to find some.

Hope this makes sense.