The measure of how much a lens can bend light as it passes through. The higher the refractive index of a lens, the slower light travels through it. By increasing the refractive index, you enable the lens to bend light more efficiently, allowing the lens to be thinner, lighter, and aesthetically more pleasing.
The refractive index of a lens determines how well light can be refracted when passing through it. Higher lens powers require higher refractive indices, allowing for a thinner, lighter and more aesthetically pleasing lens with better performing optics.