Each letter maps to the one 13 positions ahead. Since the alphabet has 26 letters, applying ROT13 twice returns the original!
ROT13 stands for "rotate by 13 places" — each letter in the alphabet is replaced by the letter 13 positions ahead.
Only letters A-Z and a-z are affected. Numbers, spaces, punctuation, and symbols pass through unchanged.
Because the English alphabet has 26 letters, applying ROT13 twice gives back the original text — encode = decode!
ROT13 is commonly used for hiding spoilers and puzzle answers online. It provides no real security — it's just obfuscation.