The Glycemic Index is a relative ranking of carbohydrate in foods according to how they affect blood glucose levels. It uses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood sugar.
Carbohydrates with a low GI value (55 or less) are more slowly digested, absorbed and metabolised and cause a lower and slower rise in blood glucose and, therefore insulin levels.
Carbohydrates with a higher glycemic index (GI more than 70), such as a baked potato, cause a large and rapid glycemic response; you may feel an initial elevation in energy and mood as your blood sugar rises, but this is followed by a cycle of increased fat storage, lethargy, and more hunger!
Scientific evidence has shown that individuals who followed a low-GI diet over several years are at a significant lower risk of developing Type-2 diabetes, coronary heart disease, and age-related muscular degeneration.
On the other hand, high blood glucose levels or repeated glycemic spikes following a meal may promote the above diseases by increasing systemic glycemic stress.
Also, it's not Glycemic Index alone that leads to the increase in blood sugar. Equally important is the amount of the food that you consume. The concept of Glycemic Index combined with total intake is referred to as "Glycemic Load". Therefore, you can control your glycemic response by consuming low-GI foods and/or by restricting your intake of carbohydrates.