"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Volvo S60, if you buy parts for your car that are not actually made by Volvo, then those are termed "aftermarket". Many people use the term "S60 aftermarket parts" specifically when referring to custom parts for the S60, like performance parts or custom appearance parts. 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 parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".