The ROT13 Caesar cipher is a letter substitution algorithm originally used to protect messages from the eyes of others. The alphabet is replaced with 26 different letters, which are rotated by 13 letters to encrypt messages. This means, if A is replaced by N, then B will be replaced by O, and so on. To decrypt messages, one would simply rotate the message 13 letters back around the alphabet.
In other words, the ROT13 Caesar cipher is a substitution cipher in which the letters A through M are replaced by the letters N through Z and vice versa.