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