Die ratings are used to measure attributes and skills. A higher die rating signifies a greater aptitude in the character. The die rating simply defines what type of dice you are supposed to roll for a skill or attribute. For example, a d2 die rating means that you should flip a coin (a two-sided "die") while a d6 means that you should roll a six-sided die for that attribute or skill. The dice should be rolled exactly as they are listed in the die rating. Rolling a d12+d12, aka 2d12, for that die rating would not be the same as rolling a d20+d4 for the same die rating. Below are some examples of die ratings from low to high.
d2 => d4 => d6 => d8 => d10 => d12 => d12+d2...2d12 => 2d12+d2...2d12+d10 => 3d12 => 3d12+d2...Edit
The die ratings continue in this fashion from 4d12 and so on.