If you've ever driven on a highway during a heavy rain, you know about the danger of skidding.
This "waterskiing" on the road is technically called hydroplaning, and it happens when water comes between the road and your tires, causing you to lose traction and go out of control. Your car is less likely to slide around when the road is dry because there is enough friction between your tires and the road to keep you steady, even at high speeds.
But when it rains, a layer of water builds up between your tires and the road. This water interferes with the friction that helps your tires grip the road surface.
This is where the treads on your tires come in. If the road you are driving along is covered with water, the pressure of the tire against the road surface causes the water to be squeezed up into the tire treads. These treads help your tires pump water out from underneath the tire so that the rubber can be in contact with the road, thus creating the friction that will stabilize your car.
The faster you go, the more water your tires have to remove. That's why your car may not hydroplane at thirty miles per hour, but might at sixty miles per hour. In fact, at highway speeds, during hard rains, each tire must pump away about a gallon of water every second.
This is hard for tires to do if they are old and have treads that are worn down. When there isn't enough space in the tire's grooves, water can't find a place to go and so it creates a layer of lubrication, causing you to do the equivalent to waterskiing on the highway.