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