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