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