The ROT13 is a very simple letter substitution cipher that replaces each letter in a text string with the 13th letter after it in the alphabet. The ROT13 is a version of the Caesar cipher that was developed in ancient Rome. The original cipher did not have a fixed amount of displacement.
The ROT13 is its own inverse. This means that the original sequence of letter is generated is the algorithm is applied twice to a given string (given that the English alphabet is 26 letter long). Just because of its simplicity, use of the ROT13, or any variations of it, is heavily discouraged when it comes to password encryption. Although is provides some measure of security - albeit very minimal, especially when it comes to more sophisticated means of password cracking - it is mostly reserved for information that need to be inconspicuous at first, such as spoilers or puzzle solutions.
Although the ROT13 is the most widely used cipher, I have included here all 13 rotations (ROT1 - ROT13), as well as the option to rotate a sequence backwards as well. Number digits are rotated in the same fashion from 0 through 9 (and backwards), while special characters and symbols remain unaffected.
I suggest that you apply the ROT algorithm with caution and don't rely on it when it comes to sensitive data. Passwords using the ROT algorithm can become only marginally stronger, as they provide some defence only against guesswork and adversaries that are not aware of the existence and application of the cipher.