Plain English Meaning
The speed above which the tires can ride up on a layer of water instead of touching the runway. Once the airplane reaches that speed on a wet runway, the brakes and nosewheel steering largely stop working until the speed drops back down.
Full Definition
The minimum groundspeed at which dynamic hydroplaning can begin on a wet runway. For a tire with smooth or worn tread, it is approximately nine times the square root of the tire pressure in psi (Vₚ ≈ 9 × √tire pressure). At or above this speed, a wedge of water can lift the tire off the runway surface, eliminating direct rubber-to-pavement contact and the braking and steering that depend on it.