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