You take a huge parking lot (or an airport), set up a tight and twisty track out of cones and race against the clock. It’s inexpensive and a safe way to become a MUCH better driver. It is mostly about the driver and not about the car. It was funny seeing people come with expensive cars and big egos only to end up the slowest :-).
The skills you learn in autocross are car control skills that are extremely useful on the road and make you a much safer driver. On the road your car almost never exceeds its limits, and when it does you are not prepared for it. Competing in autocross allows you to get to know your car, find those limits and know how to react when you cross them. In fact, we even held car-control clinics where general public can come with whatever car they have and push their cars beyond the limits.
Aside from competing, I really enjoyed coaching other drivers, instructing in our car control clinics for teenagers and taking photos of the events.