In a nutshell, glycemic index is a gauge of how different foods affect blood sugar. When eaten alone or in large amounts, foods that rank high on the GI cause fast spikes in blood sugar, leading to a surge of insulin, a hormone that helps the body use or store blood sugar.
Some say that sticking with low-scoring foods can help you shed pounds easily (no calorie counting required) or avoid diabetes, but that's an oversimplification. While some research suggests that people who down lots of high-glycemic picks have greater health risks, this is more likely because they're OD'ing on portions and loading up on processed foods with added sugars—cookies, graham crackers, and soda tend to land near the top of the index.
Many super-healthy eats also rank high on the scale, while some less nutritious fare falls low. One great example: watermelon. It's rich in healthy antioxidants like lycopene, and yet it's higher on the GI than sponge cake!
If you're diabetic or prediabetic, consuming mostly unprocessed, low-GI foods may help you regulate your blood sugar levels. But for everyone else, focus instead on eating nutritious foods and creating balanced meals.