The glycaemic index is a way of ranking carbohydrates according to how quickly they raise blood sugar.
Each food is given a GI value between 0-100 compared to pure glucose mixed in water, the closer to 100 it is, the faster it will raise blood sugar. Foods which are close to 100 are high in sugar and cause peaks and troughs in your energy level and consequently your mood. Foods nearer to 0 raise the blood sugar slowly, releasing their energy over a longer period of time, they help you to maintain consistent energy levels.
The bottom line – if you want consistent energy levels, go for lower GI foods.