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