1. A board pulled over the water by a motorboat and ridden by a person standing up.
2. Water-skiing boards on which people stand while they are being towed by a motorboat.

1. To stand on an aquaplane and to move across the surface of water; usually, by being towed by a motor boat."
2. With reference to cars, to rise up onto a thin film of water between the tires and the road so that there is no more contact with the road or street.

In other words, if a motor vehicle aquaplanes, it is sliding out of control or skimming uncontrollably on a thin film of water on a wet road.