"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Chevrolet Ssr, if you buy parts for your car that are not actually made by Chevrolet, then those are termed "aftermarket". Many people use the term "Ssr aftermarket parts" specifically when referring to custom parts for the Ssr, like performance parts or custom appearance parts. But even if you just replace your Ssr brake pads with OEM-grade factory-style brake pads that are not made by Chevrolet, those too are aftermarket Ssr 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".