A transparent second eyelid possessed by Vulcans, evolved as protection against the fierce light of their home star, which closes by reflex and saved Spock from permanent blindness.
The inner eyelid is a feature of Vulcan anatomy that developed because of the brightness of the Vulcan sun. It closes automatically when the eyes are exposed to dangerously intense light, and because Vulcans normally have no need of it they tend to forget that it is there at all.
It is established in the original series episode Operation -- Annihilate! To free Spock from the neural parasite that has attached itself to his nervous system on Deneva, Doctor McCoy exposes him to blinding light across the full spectrum, only to learn moments later that invisible ultraviolet alone would have killed the creature. Spock emerges from the test chamber sightless.
The blindness proves temporary. The inner eyelid had shut against the glare, and within hours Spock's sight returns, much to McCoy's relief and his own mild embarrassment at having overlooked it.