The Glycaemic Index, or in the world of complicated UK spelling - Glycaemic Index, is a measurement of how much sugar (glucose) is absorbed into the blood in a fixed time period, after eating a portion of the food with a fixed quantity of carbohydrate. The glycaemic index is often abbreviated to just GI.
It is useful to be aware of the GI value of some foods as including low GI foods in your diet as much as possible can help to reduce the peak of blood glucose levels after eating. However as foods are rarely eaten in isolation the GI of individual foods is less relevant as the GI of a meal will be affected by all the foods within it . Therefore it is more useful to be aware of GI values and include low GI foods in your diet as often as possible.