Okay kiddo, so you know how when you blow air on your hand it feels kinda cool? That's because the air is rushing over your hand and pushing it up a tiny bit.
Well, cars can kinda do the same thing with the air around them. When a car drives really fast, it makes a pocket of air underneath it. This air can't go anywhere because the car is blocking it from getting out. This makes the air pressure under the car higher than the air pressure on top of the car.
When the air pressure is higher under the car, it pushes the car down onto the ground. This makes the car really stable because it's harder for it to tip over or lose control. It also helps the car go faster because it's not bouncing around as much.
This is called ground effect and race car drivers use it to go really fast and get better control. Pretty cool, huh?