It must have happened to you -- same as everybody else. There you were, on a wet surface with slick-soled shoes, and your feet slipped. Maybe you even fell down.
Your car's tires work much the same way; If they're bald and worn out, they may stick to the road fine when it's dry, but when it rains, watch out. You're asking for trouble.
When the tire-tread is only .2" deep, it's time for new tires, for a lot of reasons, the two main ones being for adequate traction in wet weather and for more protection against a puncture. Deep tire-tread lest rain run between the grooves, keeping the tire firmly on the road. And a nail or other sharp object, while it may still pentrate the tire, is less likely to do so through thick tread.
If your tires have little tread left, or are bald, when you're driving in the rain the water can form a kind of film between the tire and the road surface. When this happens, you lose most or all, control of the car.
There are tire-tread depth gauges that will let you acurately and easily check tread depth, some of them selling for only about $2. Using them is simple. The gauges are calibrated in 32nds of an inch. All you do is place the guage's flauge across two tire treads, push the probe down with your finger between the treads as far as it will go, then withdraw the gauge and read the depth. The nice thing about this is that you can check all the treads and spot any uneven wear patterns.
In general, here's what you should look for when checking your tires. Tires' treads should be worn evenly all around. Rapid wear at both edges of a tire, but plenty of tread in the center usually means the tire is overinflated. A lot of tread wear in the center of the tire, but plenty of tread on both edges, probably indicates the tires are underinflated.
Wear on one side of the tire only, feathered edges on the treads and bald spots probably indicate balance or alignment problems, and you should have a garage or a tire specialty shop check things out.
Keeping tires properly inflated is very important; most people don't bother to do it. Tires that aren't inflated correctly wear out faster, and can be dangerous to drive on.
Check tire pressures weekly with gauge. Tires should be inflated to the specs called for in you owner's manual. If the tires are new and a visual inspection of the tread shows nothing wrong when you make your weekly tire-pressure check, it's not necessary to use the tire-tread depth gauge every week -- tires don't wear that fast.
But every time you check tire pressure, give your tread a quick visual inspection for irregular wear or any foreign objects that might be present.