"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Smart Fortwo, if you buy parts for your car that are not actually made by Smart, then those are termed "aftermarket". Many people use the term "Fortwo aftermarket parts" specifically when referring to custom parts for the Fortwo, like performance parts or custom appearance parts. But even if you just replace your Fortwo brake pads with OEM-grade factory-style brake pads that are not made by Smart, those too are aftermarket Fortwo parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".