Foods are broken down into three major groups, which are fats, proteins and carbohydrates. The glycemic index (sometimes spelled: glycaemic index), or GI is a measurement taken of how carbohydrates have an effect on blood sugar levels.

The Index is calibrated from high to low and carbohydrates that break down very quickly during the digestion process thereby releasing glucose into the bloodstream rapidly are said to have a high GI. On the other hand, those carbohydrates that are slower to break down and release glucose into the bloodstream at a more gradual rate are said to have a low GI.

The GI concept was originally developed in 1980-81 by Dr. David J. Jenkins along with his colleagues at the University of Toronto, during research in discovering which foods were best consumed by people with diabetes. By categorizing carbohydrates into the Index, Jenkins was able to define which foods were more diabetic friendly and which were not.

A low GI suggests a slower rate absorption by the body of carbohydrates. This may also indicate a more efficient and greater product extraction of carbohydrate digestion from the liver and periphery.

Importantly for diabetics, a lower glycemic response usually means a lower insulin demand (although not in all cases) and may help to improve the long-term glucose control and blood lipids in the bloodstream. Also useful is the insulin index, as it provides a direct means of measuring insulin response to certain foods.

The definition of the GI of a food is the area found beneath the 2 hour AUC (blood glucose response curve) which is produced following consumption of a fixed portion (usually 50 g) of carbohydrate. The AUC of the food being tested is then divided by the AUC of a standard, which can be either glucose or white bread.

This gives two different definitions. The total is then multiplied by 100. An average GI value is then calculated using data collected from 10 human subjects.

The result for each food tested provides its relative ranking. Glucose is used as the reference food and is given a GI of 100, creating a universal standard that can be relied upon for accurate results. White brad also has a GI of 100.

Source: wikipedia

So that pretty much sums up what exactly this measure of whether a person is at their correct size, overweight, obese or conversely underweight constitutes. If you have any concerns about your own physical size and need to ascertain in more depth your actual state of health, you should consult with your doctor.