How it works
Every letter is replaced with the one a fixed number of places further
through the alphabet, wrapping back to the start when it runs past Z or
z. Numbers, spaces, and punctuation are left exactly as they are.
Why ROT13 is not encryption
A Caesar cipher has only 25 possible shifts, so it offers no real protection against anyone trying to read it — including automated tools that just try every shift and look for readable text. Its traditional use is hiding something from a casual glance, like a puzzle answer or a spoiler, not from someone actually trying to read it.
A cipher that undoes itself
Because the alphabet has 26 letters, shifting by 13 twice returns every letter to where it started. That is the whole trick behind ROT13: the same operation both encodes and decodes.