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