The Elder Scrolls are an ancient series of writings, possibly by the original Eight Divines. Their origins are wholly unknown, but they can assist those few who can read them to ascertain the future. Looking at them can make one temporarily — or, with extended exposure, permanently — blind or insane. The Dragonborn is asked to find one such scroll.
There are a massive number of these scrolls, many of which are stored in a library in the White-Gold Tower in the Imperial City in Cyrodiil. This library is operated by the Order of the Ancestor Moth, or the Moth Priests, a monastic order dedicated to the recovery, preservation, translation and interpretation of the Elder Scrolls. Over their lifetimes, Moth Priests gradually become blind from repeated readings of the Elder Scrolls; they inhibit this destructive process somewhat through spiritual and mental rituals that help them prepare for a reading. Once they become blind, they retire to the Temple of the Ancestor Moth, where they live in perpetual care until they pass on.
Apparently, the ancient, extinct Dwemer developed a method to circumvent the ill effects of reading an Elder Scroll with the use of a massive, mechanical construction that transcribed Elder Scrolls onto 'lexicons'. Then, they could be interpreted safely. One such construct exists deep in the bowels of a Dwemer ruin known as Blackreach, found in eastern Skyrim.
Found after solving the puzzle involving the Lexicon and the huge dwemer artefact.