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