The refractive index is dependent on the material, and is a measurement of the amount that that material will refract or “bend” light as it passes through. At 90 degrees or dead center of the filter the light does not deflect and continues straight, but as the angle of incidence towards the edge of the filter increases, it deflects light more and more causing a discrepancy across your frame, particularly with wide angle shooting. The higher the refractive index value, the more image quality can suffer.