"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Volvo C30, if you buy accessories for your car that are not actually made by Volvo, then those are termed "aftermarket". Many people use the term "C30 aftermarket accessories" specifically when referring to custom accessories for the C30, like performance accessories or custom appearance accessories. But even if you just replace your C30 brake pads with OEM-grade factory-style brake pads that are not made by Volvo, those too are aftermarket C30 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".