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