The GL scale is effectively an evolution of the Glycaemic Index (GI). This is a rating scale that marks the carbohydrate content of a food on a scale from 0 to 100. Foods are ranked according to how much they raise blood-sugar levels after eating - i.e how quickly they are broken down in to glucose during digestion.
High GI foods are digested and absorbed quickly, which can result in large and often harmful spikes in blood sugar levels. Low GI foods however are digested and absorbed more slowly, keeping blood sugar levels steady.
However, GI does not take in to account the amount of these foods, this is where Glycaemic Load (GL) comes in. Taking into account both the GI of a food, and the amount of carbohydrate content per 100g of this food, the GL to allows a better overall means of dietary guidance.