Glucose from simple carbohydrates is absorbed into the bloodstream quickly, whilst complex carbohydrates take longer to break down. The higher the GI, the faster it will get into your blood.
Each time you eat or drink something sugary or starchy, the blood glucose level in your body rises. Some of these are quickly digested and cause quick and sharp rises in your blood glucose levels - they are called high GI foods and drinks. Low GI foods and drinks are more slowly digested and will make your blood glucose rise more slowly. These are sometimes called ‘slow release’ carbs.
Rule of thumb is that the more fibre in something, the lower the GI.