Index of Refraction is the ratio between speed of light in a vacuum and the speed of light in a given material. The resulting number - 1.50, 1.60, 1.67 or 1.74 is referred to as the Index. The higher the number, the more light slows down or bends in a given material. This means a high index lense bends light at steeper angle than a standard lower index lense.
Therefore, high index lenses need less curvature to achieve the same focal power. That is why a lens made from a 1.67-index material can be quite thinner than one made from a standard 1.50-index plastic material with the same prescription.