ROT-13 is the encrypting of a message by exchanging each of the letters on the first half of the alphabet with the corresponding letter in the second half of the alphabet (that is, swapping positions by 13 characters). Thus, A becomes N, B becomes O, and so forth, and conversely, N becomes A, O becomes B, and so forth. Numbers, spaces and punctuation are not changed. ROT-13 is sometimes used to encrypt messages that may be offensive or of questionable taste, or messages that contain spoilers (like movie endings or punch lines). The purpose of the code is not to guarantee security, but simply to make it difficult for anyone to read.
For more details : https://searchsecurity.techtarget.com/definition/ROT-13