The performance of your car is called Performance Indicator (also known as P.I.). It is the sum of all the statistics from your r&d/car setup. There are two types of stats: Mechanical stats and Driver stats.

Mechanical stats

• Power: Improves the maximum achievable speed of the car.
• Aerodynamic: Decreases air resistance and improves the cornering speed of the car.
• Lightweight: Decreases the weight of the car, improving acceleration.
• Grip: Improves the speed carried through slow, medium, and high-speed corners.

Driver stats

• Braking: Increases deceleration during braking, allowing you to brake later.
• Overtaking: Increases the effectiveness of slipstreaming improving your overtaking ability.
• Adaptability: Improves your grip during adverse weather conditions.

Some types of parts only affect Mechanical stats whereas others only affect Driver stats. For more details, please check out this article: "What are the different types of Parts and what makes them different and unique?".