Based on a scale of 1 to 100, the Glycaemic Index (GI) is simply a ranking of carbohydrate foods according to the rate at which they raise blood glucose levels. 

When you eat foods such as bread, potatoes, pasta, cereals, sugary treats etc, the body digests the carbohydrates and breaks them down into glucose which can then pass into the bloodstream to provide energy. 

Foods that release glucose into the bloodstream rapidly have a high GI score, whilst foods that take longer to digest, releasing glucose more slowly, have a low GI score.