GI = Glycemic Index. A low GI diet is one in which the things you eat will not spike blood sugar levels, but will work to help stabilize them.
For instance, eating a piece of candy will spike your blood sugar rather quickly, whereas eating a carrot will not, because of the lower sugar/carb content. Regular pasta will spike your blood sugar more quickly than whole gain pasta because it takes longer to digest the whole grain, so it doesn't go straight to the blood.
A low GI diet is good for everyone, but particularly so, for diabetics who take medication based on blood sugar levels.
Here's a link that explains it pretty well or you can google "low glycemic foods" to come up with a lot of sites that will tell you which foods are lowest in the glycemic index.
http://nutritiondata.self.com/topics/glycemic-index