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