The glycemic index of a food is a measure of the rate at which the carbohydrate from food is broken down into glucose in your bloodstream. While choosing low GI foods has many benefits, the overall effect that a food has on blood glucose levels is dependent on both the nature (GI) of the carbohydrate it contains and the amount (ie grams of carbs). Glycemic load takes both of these factors into account..
While foods with a high carbohydrate content and those with higher GI values will generally have the highest GL, this also means that small amounts of a high GI food may have only modest effects on blood glucose levels while, as Courtney points out above, large amounts of a low GI food can still raise blood glucose and insulin levels significantly. What this means in practice, is that there is no need to avoid foods that have a high GI but are low in carbohydrate and nutrient-dense. On the other hand, just because a food is low GI, it doesn’t mean you can eat as much as you like, particularly if you are watching your weight or your blood glucose or insulin levels.