If a serving of food was a car, the Glycemic Food Load (also glycaemic load, GL) would be the gas in the tank and the glycemic food index (GI) would be the engine power.
Imagine if you will driving a moped down the highway with a big 10 gallon gas tank strapped to the back. You're not getting anywhere too quickly, but you sure can go a long distance on that thing.
This is the same as eating a low GI, high GL food. The glucose enters the blood slowly, but there's lots of it and it's delivery is spread out over time. Foods that are high GL and low GI are excellent for endurance athletes who need a lot of energy and don't want to "bonk."
Now picture yourself with the same 10 gallon gas tank, but instead of a moped you're peeling the tires of a Porche 911 down the autobaun. That gas isn't going to last too long now is it?
Its the same way when you eat a high GI food. The glucose enters the blood quickly giving you a "sugar high", followed by the subsequent "crash" when the glucose runs out. No matter how much gas in the tank, that Porche is going to burn it quickly.
If you're trying to lose weight, its a good idea to eat mostly low GI, low GL foods. That way you're not overloading your body with too much carbohydrate. And you're spreading what carbohydrate is available over the longest period time leaving you feeling full and energetic for longer and less inclined to overeat.
Simply put, while glycemic index is the rate at which food is converted into glucose, glycemic load is the total amount (load) of glucose provided by the food. Glycemic index is an absolute value, while glycemic load depends on the serving size of the food in question. Glycemic load values are always quoted in reference to a serving size in grams.
GI and GL are mathematically related to each other by the amount of available carbohydrate in a food. See How to Calculate Glycemic Index for more information on the relationship between GI and GL.