If you go to the top of the page, click on 'Data,' and pick the year model of your car, it'll have the VIN Code deciphering information listed on a new page. The information found there can help make a ball park determination as to the actual value of a vehicle based on its equipment package.
The letter codes indicate engine package, as Don said. Those differences can certainly explain value and rarity, for instance, an R-Code (Boss 351) will command more money (in any condition) than an H-Code (351C-2V), because of less numbers produced in addition to being on the top-end of the performance models - an H-Code, on the other hand is not as valuable because it's "just" a 351 Cleveland with a 2bbl carb/intake.
It's helpful when determining what kind of a car you want in the long run. If you want a restomod car (like mine), you're better off getting a lesser valuable model and making it your own - that way you're not paying premium price for a collector's car just to have a fun hot rod you can drive the piss out of and enjoy.
If you want something that will definitely increase in value, then hold out (and save your pennies) for a nicer, more valuable car, put it on a trailer, get a few trophies, and sell it for a profit in a few years when you get bored just staring at it in the garage.
