Speed of the rise versus the size of the impact
Glycemic index and glycemic load answer two different questions. Glycemic index describes how fast a given amount of a food's carbohydrate pushes blood glucose up, essentially the speed and steepness of the rise. Glycemic load combines that speed with the actual quantity of carbohydrate in a portion to estimate the overall impact a real serving has on blood glucose.
A food can rate high on one and low on the other, which is why using only one number can mislead.
Worked example: the watermelon paradox
Watermelon illustrates the split cleanly. Its sugars are quickly absorbed, giving a high glycemic index, yet a normal slice contains relatively little carbohydrate because the fruit is mostly water, so its glycemic load is low. Judging it by index alone would overstate its effect on blood sugar; judging by load reflects the modest reality of a typical portion.
This is general educational information, not medical advice. These indices are population averages and do not capture individual variation; anyone managing diabetes should plan with a qualified professional.