A tire's traction grade indicates how well a tire can stop on a wet paved surface without skidding off to a side.  There are four possible traction grades: AA, A, B, and C.  A rating of AA is the highest possible traction grade, while C is the lowest. 

 

Traction grades are determined by driving a vehicle on wet pavement "skid pads" provided by the U.S. government.  At 40mph the wheels are locked,  thus placing the entire stopping burden on the tire.  

 

A tire's traction rating only applies to stopping in the test described above.  It does not indicate a tire's stability during cornering or at high speeds.