Hydroplaning happens when a layer of water builds up between your tires and the road faster than your tread can channel it away, and your tires briefly lose contact with the pavement entirely. It usually happens without warning: the steering wheel suddenly feels light, the car drifts, and pressing the gas or brake seems to do nothing. It only lasts a second or two, but that's long enough to cause a serious accident if you react the wrong way.
The most common warning sign is the steering wheel going strangely light or unresponsive, almost like power steering just cut out. You might also hear the engine rev slightly as the drive wheels spin freer than the road speed suggests they should. It typically happens at higher speeds, in standing water, or right after rain starts, when oil and debris haven't washed off the road yet.
Slow down in heavy rain, especially in the first 10 minutes after it starts. Avoid standing water when you can see it, and stay out of the outer lanes where water tends to pool. Worn tires make hydroplaning far more likely since there's less tread to channel water away, so check yours regularly. Keeping enough space ahead of you also matters, more following distance gives you room to react calmly instead of needing to brake hard the moment you feel the car shift.