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