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 Japanese fast cars "tuners", comparing to American fast cars. It is because people call American fast cars "muscles".
When people want to drive fast cars but don't want to buy a very expansive car, we can change some parts for an ordinary car to make it go fast.
[change] Making a Tuner
For basic tuning, we can work on intake and exhaust, which means the process that air goes into the engine and out of the engine. We can also make the frame of the car stronger.