RPO is an acronym for
Regular
Production
Option. Options could be the style of wheels, body color, heated seats, particular engine, etc. They're all listed on a sticker where
@luxuryrules indicated. Or as
@Submariner409 said, you can get a build sheet that lists all of the RPO codes for your specific VIN, along with the meaning of the code (see below). Some websites offer them for sale. The parts department at your local dealership may also provide you one, often for no fee. There used to be a way to get one via email directly from GM, but they ended that service a couple of years ago.
Each code has a specific meaning. Not specific to your vehicle, but (generally) specific within all GM vehicles. Every once in a while some codes may change meaning, but they're pretty consistent. Using the RPO code is an easy way to reference something - for example, there have been discussions where the suspension is the topic and people will refer to RPO code FE
x, where
x is some number indicating the type of suspension.
This is a snippet from the build sheet for the wife's '13 ATS. The three character alphanumeric values (e.g. 1SU and 4C2) are the RPO codes that define the features of her vehicle.