"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your BMW X6, if you buy parts for your car that are not actually made by BMW, then those are termed "aftermarket". Many people use the term "X6 aftermarket parts" specifically when referring to custom parts for the X6, like performance parts or custom appearance parts. But even if you just replace your X6 brake pads with OEM-grade factory-style brake pads that are not made by BMW, those too are aftermarket X6 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".