Glycemic index is a comparative measure of the effect any food has on the blood sugar. The comparative here means that the effects of the food will be measurable relative to some reference food, which is usually, arbitrarily, assigned to either white bread, or, more often, glucose. You can easily convert between the two.
Of any food only a fraction is in the form of carbohydrates and of that only a fraction has glycemic effect. The carbohydrates in the food that elicit changes in blood sugar are called available carbohydrates. E.g. one serving size, 120[g], of "Apple, Golden Delicious (Canada)" contains only 16[g] of available carbohydrates.
How many of those 16 grams of available carbohydrates will affect blood glucose levels is determined by the Glycemic Index of the food. For "Apple, Golden", the GI=39
Product of the amount of available carbo-hydrate in a specified serving size and the GI value (using glucose as the reference food), divided by 100 yields Glycemic Load
Taking this into consideration 1 unit of Glycemic Load in food will be equivalent to ingesting 1[g] of glucose, or 1.41[g] of white bread.