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It is an indicator of tire tread wear.
you could put a lincoln penny in the tread groove, with lincoln head facing up, if you could see all of the top of lincoln's head its time to replace the tire.
Always check for visable damage to the tire itself. If there is no visible damage, the easiest way to check tire wear is the penny test. Place a penny "head down" into the groove of a tire, If you can see the top of it's head, your tires need replace, legally. However, you should replace your tire when the tread is reduced below 4/32 of a inch. The easiest way to measure this is actually to us the "Quarter test". Do this test the same as the penny test on with a quarter, for a more accurate reading or check your tires "wear bars" which are bars within your tread that are below the tread and are the minimum safe depth for your tire.
If you mean tire tread depth, the minimum is 1.6mm.
Minimum LEGAL tire tread depth is 2/32nds of an inch.
Tire spec in PA is 2/32 of an inch of minimum tread
1 mm on whole tire. If any part of tread is less then it should be replaced.
The tread is what grips the road.
Tire spec in PA is 2/32 of an inch of minimum tread
The outside of a tire consists of the tread and sidewalls.
Track is the dimension from the centerline of the tire tread on one tire to the centerline of the tire tread on the opposite tire on the same end of the vehicle, front track or rear track.