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