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