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