ROT13 is a simple cipher used to obscure geocache hints. Geocachers who want the hint can decrypt it fairly easily by hand, but it is more difficult for others to "accidently" read the hint (and spoil their fun).
Here is the key for ROT13 encryption/decryption:
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 encrypt or decrypt a letter, just find it in the key and select the letter above or below it.
Ybbx va gur gerr
The first letter of this encrypted hint is Y, which corresponds to L in the key. So we know that the hint starts with L:
L Ybbx va gur gerr
The next two letters are bb, and b corresponds to o. Then x corresponds to k, and we have the first word of the hint:
Look Ybbx va gur gerr
We can decrypt the rest of the hint in the same manner:
Look in the tree Ybbx va gur gerr
Many experienced geocachers have memorized the ROT13 key and can decrypt caching hints very quickly. Also, certain words (e.g., "magnetic") appear so frequently in caching hints that many experienced geocachers recognize them in their encoded form (e.g., "zntargvp").