It's called Bernoulli's Principle, named after Swiss physicist Daniel Bernoulli who literally wrote the book on the subject way back in the 1700s. Countless engineers have spent their careers focused on its study and application, but the crux of the matter is that, as the speed of air (or other "fluid") increases, pressure decreases. Play with the air's increasing speed and decreasing pressure just right and you can generate downforce underneath the body of a car without significantly increasing drag as you would with surface spoilers.
For evidence of how Bernoulli's Principle applies in practical terms, just look at the last Ferrari to pack a turbocharged V8 in the middle and the latest one. The F40 had a giant wing on the back, where the 488 GTB has none. But because the 488 uses underbody aerodynamics (or "ground effects"), it generates significantly more downforce than the winged F40 ever could, and at lower speeds.
Ferrari, however, was not the first outfit to harness the power of ground effects. Lotus did with the legendary 79 that Mario Andretti drove to the world championship back in 1978. That was the genius of Colin Chapman, and to explain how it all works in layman's terms, our friends over at Goodwood Road & Racing brought in Colin's son Clive Chapman, head of Classic Team Lotus, to put together the video above.