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