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 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.
If you are only interested in a region, the command M-x rot13-region might be preferable. This will encrypt/decrypt the active region in-place. If the buffer is read-only, it will attempt to display the plain text in the echo area. If the text is too long for the echo area, the command will pop up a temporary buffer with the encrypted/decrypted text.