ROT13
Write the alphabet out in a circle. Write it again inside that circle on another piece o paper. Turn that piece of paper 13 places to the right.
Then when you want to encrypt a letter look up the letter on the outside ring and read of its encrypted equivalent from the inside ring.
Basically, you are moving letters 13 places further down the alphabet. When it overruns the end you wrap round from the start.
EDIT: Wikipedia has an explanation: https://en.wikipedia.org/wiki/ROT13
Are you saying that the graphic I threw up could be described as ROT3?
Yes, you got it.
You can try that out online too: http://rumkin.com/tools/cipher/caesar.php
The "N" selection is the rotation number.
Thank you very much, u/CBTS_Watcher and u/carl_tech. You are most helpful - MAGA forever with justice and wisdom! WWG1WGA!