The index of refraction (or refractive index) of an eyeglass lens material is a number that is a relative measure of how efficiently the material refracts (bends) light, which depends on how fast light travels through the material.
Specifically, the refractive index of a lens material is the ratio of the speed of light in a vacuum, divided by the speed of light in the lens material.
For example, the index of refraction of CR-39 plastic is 1.498, which mean light travels roughly 50 percent slower through CR-39 plastic than it does through a vacuum.
The higher the refractive index of a material, the slower light moves through it, which results in greater bending (refracting) of the light rays. So the higher the refractive index of a lens material, the less lens material is required to bend light to the same degree as a lens with a lower refractive index.
In other words, for a given eyeglass lens power, a lens made of a material with a high refractive index will be thinner than a lens made of a material with a lower refractive index.
The refractive index of current eyeglass lens materials ranges from 1.498 (CR-39 plastic) to 1.74 (a specific variety of high-index plastic). So for the same prescription power and lens design, a lens made of CR-39 plastic will be the thickest lens available, and a 1.74 high-index plastic lens will be the thinnest.