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