Regular plastic eyeglass lenses block most UV light, but adding a UV-blocking dye boosts UV protection to 100% for added safety. Other eyeglass lens materials, including polycarbonate and most high-index plastics, have 100% UV protection built-in, so an extra lens treatment is not required for these lenses.
Just as sunscreen keeps the sun's UV rays from harming your skin, UV-protective treatments for eyeglass lenses block those same rays from damaging your eyes.