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