If you've ever shopped for tires, you've probably noticed the letters H and T on the sidewall of some tires. These letter designate the speed rating and load index of a tire, respectively. Speed ratings range from L (the lowest) to Y (the highest), while load indices go from 70 (the lowest) to 126 (the highest).
The speed rating tells you the maximum speed at which a tire can carry a load under specified conditions. The load index is simply a number that corresponds to the maximum weight that tire can support when properly inflated.
So, if you see a tire with an H speed rating and a load index of 104, that means it can safely carry up to 1521 lbs at 130 mph.