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