Mailing list messages that might offend or annoy some readers are sometimes encoded in a simple code called rot13—so named because it rotates the alphabet by 13 letters. This code is not for secrecy, as it provides none; rather, it enables those who wish to to avoid seeing the real text of the message. For example, a review of a film might use rot13 to hide important plot points.
To view a buffer that uses the rot13 code, use the command M-x rot13-other-window. This displays the current buffer in another window which applies the code when displaying the text.