A tire's tread is measured in increments of 1/32 of an inch.  In the United States, tires are legally worn out when a tire's tread is 2/32 inches deep.

 

A treadwear indicator is a small bump raised 2/32 inches, located between a tire's treads.  When the treadwear indicator is even with the tire's tread, your tires need to be replaced.