ROT13 stands for “Rotate by 13 places” which fits well as it is a very simple substitution cipher uses the Caesar cipher method of shifting letters along the alphabet. The difference is that where Caesar cipher has a key (the number of characters to shift), ROT13 can only shift each letter 13 characters along.
As the Latin alphabet has only 26 letters it means that the same algorithm for encryption can be used for decryption, making it very unsecure. It provides little to no security and is typically used as an example of poor encryption.