This feature is called the wear bar/wear indicator ... and the base of the gauge on the top of the tread. Then, read the measurement where the scale stops at the top of the gauge.

The result is hydroplaning, where the tread skims the water's surface and the vehicle no longer responds to the steering wheel. Wet-weather braking and snow traction also decrease as tires wear.

Note that shaving a tire will likely nullify its tread-wear warranty. Other retailers may offer a similar service, though the special equipment to shave a tire’s tread isn’t common.