In most parts of the US and the world at large, tire tread is measured in 32nds of an inch. When the tire is new, the tire tread is about 10/32”. If a tire is worn out, it can be dangerous to drive. It is because a worn out tire lacks proper grip on the road.
Apart from the tire indicators used to check the tire tread, you can see also the quarter to check the tire tread. You can take your quarter and insert the head into one of the tire grooves. If the top of the head is visible, then you should not hesitate to buy a new tire. You should know that the tire is worn out, and continuing to drive using these tires can be dangerous. Apparently, nice tires are efficient when one is making emergency brakes. For instance, you may be driving at high speed, and an animal crosses the road, it would take long for the car to stop if the tread is not in good condition.