In optics, the refractive index or index of refraction of a substance or a medium, is a measure of the speed of light going through the medium. Or how much the speed of light is reduced inside the material that the lenses are made of.
There are two common qualities to glass and other transparent materials witch are directly related to there refractive index.
Light rays bend and change direction when they cross the interface between air and the material, an effect that is used in corrective (Ophthalmic) lenses, to concentrate light into the focal point.
Light partially reflects from surfaces that have a different reflective surface from that of there surroundings.
Lenses that are made with material that has a higher refractive index are going to be thinner, lighter, and flatter than lenses made from material with lower refractive index.
Lenses with higher refractive index have one main downside: the higher the refractive index, the higher the need for Anti Reflective coating on the lens(AR).