Hydroplaning happens when a layer of water gets between your tires and the road surface. The vehicle loses traction, the steering feels light, and the brakes don't slow the car effectively.
Tire tread depth is the primary defense. New tires (10/32") channel water aggressively through deep grooves. Worn tires (4/32" or less) can't move enough water and start skimming.
Speed compounds the problem. At 35 MPH, even moderately worn tires usually maintain contact. At 65 MPH, the time available for water to escape from under the tire drops dramatically, and even decent tires can start hydroplaning.
If you start to hydroplane: ease off the gas (don't brake hard), keep the steering pointed straight, wait for grip to return. Sudden inputs make the situation worse.
Tires that are 4/32" or below should be replaced before the rainy season. The cost of replacement is far less than the cost of a hydroplaning-induced crash.