The wikipedia article above can provide you with all the details and mathematics about ROT13, here is why it relates to geocaching.
ROT13 is a simple cipher used to obscure geocache hints in a way that allows a determined person to decode the hint if they want to, but makes it impossible to "accidently" read the hint (and spoil your fun). ROT-13 can be decoded by hand with just a pencil rather quickly. Here is the key:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
To encode or decode a letter, just find it on the chart and pick the letter above or below it as your coded letter.
Many experienced geocachers have memorized this chart and can decode caching hints in a flash.