Hydroplaning is when

Hydroplaning occurs when a tire encounters more water than it can scatter. Water pressure in the front of the wheel pushes water under the tire, and the tire is then separated from the road surface by a thin film of water and loses traction. The result is loss of steering, braking and power control.

The roads are really wet and in a car sometimes you can lose traction with the ground and start to ride on top of the water. This is dangerous and happens when you're going to fast or on a turn and there's a lot of water on the road.