"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Buick Rainier, if you buy parts for your car that are not actually made by Buick, then those are termed "aftermarket". Many people use the term "Rainier aftermarket parts" specifically when referring to custom parts for the Rainier, like performance parts or custom appearance parts. But even if you just replace your Rainier brake pads with OEM-grade factory-style brake pads that are not made by Buick, those too are aftermarket Rainier parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".