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