"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your BMW X1, if you buy accessories for your car that are not actually made by BMW, then those are termed "aftermarket". Many people use the term "X1 aftermarket accessories" specifically when referring to custom accessories for the X1, like performance accessories or custom appearance accessories. But even if you just replace your X1 brake pads with OEM-grade factory-style brake pads that are not made by BMW, those too are aftermarket X1 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".