A car acts like an (imperfect) airplane wing at speed-the pressure difference between the air flowing around the top of the car and the air going underneath can lift the car up. This is why you have airflow spoilers on the lip of the trunk of certain cars, and actual upside-down wings on struts on higher-performance ones.

But...those create drag, so there's diminishing returns in using them. Another option is to disrupt or remove the air flowing under the car. This can be done by passive airflow redirection using an undercarriage tray, or actively via fans/blowers, or both.

F1 designer Gordon Murray famously developed a racecar using a fan duct (the '78 Brabham BT46) which did so well it was immediately banned. He later went on to do the McLaren F1 road car, as well as various recent GMA sports cars with this feature.