"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Volvo S60, if you buy accessories for your car that are not actually made by Volvo, then those are termed "aftermarket". Many people use the term "S60 aftermarket accessories" specifically when referring to custom accessories for the S60, like performance accessories or custom appearance accessories. But even if you just replace your S60 brake pads with OEM-grade factory-style brake pads that are not made by Volvo, those too are aftermarket S60 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".