A tire wear indicator is a tiny nodule, or bump in the tread groove of your car or truck’s tires. This wear indicator is used by mechanics to properly gauge the thickness or depth of your tire’s tread. Every tire has wear indicators in the tread groove. On some tires, especially those with extra deep tread, the wear indicator may be difficult to see until the tire has worn quite a bit. A mechanic will place the measuring tip of his/her tire tread depth tester on the top of a wear indicator and push down on the barrel of the tester, reading the side of the barrel to check the depth of the tread.
For more info, see 3 Causes of Tire Wear and Tear