I really want to go find some letter boxes today. There's this one box I've been wanting to find and the clues are all letters with three digit numbers behind them. I know nothing about GPS or anything like that so I'm wondering is that the only way to solve this. Or is this related to something else that I am NOT aware of. Thanks

If it has two three-digit number then it may be the last three digits of the GPS coordinate of the letterbox, especially if those letters are N and W. However, you didn't say how many sets of letters followed by digits are there.

One possible meaning of this is a book cipher. This is the cipher used in the movie National Treasure, but it was called "Ottendorf cipher". The problem is finding the key (which isn't always a book) used to code the message. (Perhaps the letters are a coded message indicating the key.) I have made two letterboxes using a book cipher where the key isn't a book but is a monument indicated on the maps provided.

You can also check out the Beale ciphers.

More likely the message is using some other code. There should be some hint on the page somewhere, perhaps in the title of the box.

My #1 son taught me that clue writers will (very often) give you all the information that you need to solve the puzzle.

Are there any hints within the clues as to how to solve the number code?