How does a spoiler in a car work ?
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I’ll start by clarifying that there’s a difference between a wing and a spoiler, and they don’t do the same thing. The first two responses got it wrong, but it’s something that is commonly misunderstood, so they’re not alone.
Wings are used to push the car into the ground, which generates more friction on the tires, increasing grip/traction. This is “aerodynamic downforce.” Essentially, an airplane wing turned upside down. Wings generally stand off the bodywork of the car.
Spoilers work to disrupt air flowing close the car in ways that aren’t efficient or desirable, including turbulent air near the car that causes drag, reduces overall downforce, and/or too much noise. One key location for spoilers is the rear, where the spoiler works to free the air swirling around the rear window, and at the back of the the trunk that would otherwise pull backward on the car (like eddies in a river).
Another way to think of it is that, by messing-up the air that’s close to the car, the air flows around the car are much smoother (“laminar”), so the car performs better. Spoilers are all over the car, too – in the front air dam, on the underside of side mirrors, etc. Spoilers can generate their own downforce too, but its often secondary to the job of disrupting other bad air. Spoilers are generally closer to the bodywork than wings, and are often molded into the trunk lid and fenders.
Does that answer your question, or are you asking also about how wings and spoilers actually use air to create negative lift (wings) or reduce/disrupt/increase/redirect air and air pressures?
[edit] I thought of another, more ELI5 way to describe what a spoiler does: think of a river flowing over a hole. The water will want to fall into the hole, but since there’s water in the hole already, it’ll push some of that hole water out and up in a rough way, mixing with the smooth water and swirling around (turbulence), causing the water flowing around it to slow down. But if you were to add a flat stick (spoiler) before the hole, it’ll kick water up and away from the hole, smoothly (laminar), so it passes over the hole faster (with less drag). A spoiler works in a similar way on a car.
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