What is the Glycemix Index?
The glycemic index is basically a measurement of the effects of carbohydrates on the blood glucose level of an individual (measured over a 2-3 hour period).
Measurements are based on a scale of 0 through to 100, with 100 being the
base score for pure glucose. Foods are generally classified into 3 different
groups - low (<56), medium (56-69) and high (>69). 
Typically foods with a high GI score will spike the blood sugars levels quite quickly, and then will decrease quite rapidly. This can cause a person to feel hungry again within a short period of time. A by product of high GI foods is the increased production of insulin.
Examples of foods with a high GI include potatoes, jasmine rice and white bread
A food with a low GI score however, will not raise the blood sugar levels as much and will also take a longer time to reduce. This can stop a person feeling hungry again.
Examples of foods with a low GI score include mushrooms, apples and grainy breads.
But that's not thew whole story. There's then the Glycemic Load. The glycemic load also takes into account the amount of carbohydrates within a serving of a particular food. it is calculated by multiplying the GI score of a food by the amount of carbohydrate per serving and dividing by 100.
So for example, even though watermelon has a high GI score of 72, a 150g serving only contains 8 grams of carbohydrate, giving a GL value of 5.76 (72 x 8 divided by 100). An apple which has a low GI value of 34 has a GL value of 6.12.
So you can see that it is not necessarily the GI value that counts, but more so the GL value in maintaining a healthy life style.