When you buy a toy, sometimes it comes with extra pieces that you can add to it to make it look cooler or work better. They might not be part of the original toy, but you can add them later if you want. The same thing happens with cars. Sometimes people want to make changes to their cars after they buy them. They might want a different color, different seats, or a better sound system.
The things that you add to a car after you buy it are called "aftermarket" parts. Think of them like accessories for your car. Aftermarket parts can be anything that is added to or replaces something on the original car. They can be things that make the car more powerful, like a bigger engine or better exhaust system. Or they can be things that make the car more fun, like a new radio or cool headlights.
People like aftermarket parts because they can personalize their car and make it unique. But sometimes aftermarket parts can cause problems. If you add something to your car that it's not designed for, it might not work as well or could even be dangerous. That's why it's important to be careful when choosing aftermarket parts and to make sure they are designed specifically for your type of car.