Glycemic index is a measure of how different foods affect our blood sugar levels, as a function of carbohydrate content of the food, and the speed with which it is broken down to glucose (which is the carbohydrate energy 'currency' our body works with) and released into the blood stream.
Quick release of glucose causes a spike in blood sugar levels, which alters the natural balance of our blood, causing our body stress. In order to minimize this stress our body attempts reaching balanced sugar levels again by massive release of insulin from the pancreas (a hormone responsible for transporting glucose into cells). Some short term issues related to this pattern of insulin release are that cells receive an excess of glucose and are unable to use it all for energy, and so, to keep it for a 'rainy' day; they convert it into fats, retaining it as fatty tissue. On top of that, the stress that brought about insulin release is such that the body often cannot quantify the exact amounts of sugar and calibration of insulin release is not precise. This results in an excess of insulin release (in attempt to clear it from blood stream where it can become toxic), paradoxically leading to a low blood sugar as too much has been cleared away from the blood, which our body translates back to us as hunger, specifically for something sugary.
Long term effects of foods that have high GI (Glycemic Index), i.e. readily available carbohydrates and sugars, are that the process for clearing blood sugar into cells slowly gets overused and desensitized (the insulin receptors on cells literally become desensetized to it). The body responds to that by excreting ever growing amounts of insulin in order to overcome lack of sensitivity, which in turn places insulin excretion also at risk of becoming overused by putting too much pressure on it/ These processes encourage many disease states in the body, the most direct of which is diabetes.
The higher the Glycemic Index value of a food - the higher its breakdown and absorption into the blood (this is the bad kind)
The lower the Glycemic Index value, the slower it is digested and released into the blood stream which supports a slow and constant upkeep of a good blood sugar level for many hours. This effect creates a longer satiation effect from the food and prevents creation of adipose (fatty) tissues as a response to quick sugar breakdown.
High GI value is anything upwards of 70
Medium GI value is between 55 - 70
Low GI value is anything below 55.
The GI value of glucose is 100. White table sugar has a GI value of 68 and brown sugar is not much different to that as it is usually just as processed as white sugar, or is even just white sugar with a bit of added molasses.