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