Lens index refers to the measurement of how efficiently a lens can bend light. It is a numerical value that indicates the thickness of the lens based on the prescription. Higher lens indexes indicate thinner and lighter lenses, while lower indexes indicate thicker and heavier lenses. Lens index is important because it determines the overall thickness and weight of your eyeglasses. By choosing the appropriate lens index based on your prescription, you can ensure that your eyeglasses are comfortable to wear and provide you with clear vision.