The glycemic index is a rating system that classifies foods based on how quickly they cause increases in blood sugar levels, with a scale of 1-100. Foods with a high glycemic index of 70 or above break down rapidly, while low glycemic foods below 55 break down slowly. The glycemic load takes into account both the glycemic index and amount of carbohydrates in a food. Consuming moderate to low glycemic index foods using an 80/20 rule is recommended to control blood sugar and insulin levels and prevent health issues like diabetes.