Temperature grades are an indication of a tire's resistance to heat.
Sustained high temperature (for example, driving long distances in hot weather), can cause a tire to deteriorate, leading to blowouts and tread separation.
From highest to lowest, a tire's resistance to heat is graded as “A”, “B”, or “C”.

Of current tires:

27% are rated “A”

59% are rated “B”

11% are rated “C”