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.
Ybbx va gur gerr
The first letter of this encrypted hint is Y, Y corresponds to L on the key above. So we know that the hint text starts with L:
The next two letters are bb. Using the key above, we see that they become oo. x becomes k, so the first word is:
As we continue to decode the hint, eventually we come up with this:
Look in the tree.
Many experienced geocachers have memorized this chart and can decode caching hints in a flash. Also, certain words (e.g., "magnetic") appear so frequently in caching hints that many experienced geocachers can recognize them in their encoded form (e.g., "zntargvp").
rot13.com: contains an online deciphering program for Rot13 encryption
c1ph3r 1.3: freeware Palm OS software for Rot13 encryption, reversing text, or converting text to l33t speak.