Tuner (car)

When talking about cars, a tuner is a car with many parts changed so the car can go very fast. The word "tune" here means "to adjust" or "to change". Typically, people call any vehicle that has been modified for performance reasons a tuner car regardless of make or model.
When people want to drive fast cars but don't want to buy a very expensive car, they change some parts for an ordinary car to make it go faster.
Making a tuner.
For basic tuning, the engine intake and exhaust are changed, which means the process that takes air into the engine and out of the engine happens faster. People can also make the frame of the car stronger, to help it go around corners better. A spoiler or wing helps air push the car down onto the ground, which means more grip.