"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your GMC Acadia, if you buy parts for your car that are not actually made by GMC, then those are termed "aftermarket". Many people use the term "Acadia aftermarket parts" specifically when referring to custom parts for the Acadia, like performance parts or custom appearance parts. 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 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".