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