From the Wiki:
Fantastic artifacts of unknown origin and quantity, the Elder Scrolls (whence the series takes its name) are, simultaneously, archives of both historic and future events. Given the appellation of "The Aedric Prophecies" by the First Era sage Therin of Mournhold, the scrolls are more appropriately entitled Elder Scrolls.
Invoking power through a series of complicated rituals, a mystic of incredible strength is granted the ability to interpret the contents of the Elder Scrolls, which would otherwise appear as mere blank parchment. While certain fated individuals have been known to read the Elder Scrolls without the benefit of the associated rituals, the information revealed to both them and the mystics is never absolute. That having been said, once an event foretold within the scrolls is carried out in the world it becomes fixed within them, a part of prophecy no longer, but one of history, unable to be removed from the scrolls through any invocation of power or magic.
Such insight into the inner fabric of reality comes at a price, however, as each new foretelling and interpretation strikes the reader with blindness for a greater period of time, while simultaneously granting them a broader view of the scroll's contents. Ultimately, the reader, having engaged in frequent acts of prophecy, is left bereft of their vision, forever after removed of their right to read the scrolls. By time-honored tradition only those of The Cult of the Ancestor Moth may read from the scrolls, the younger members caring for the elder as they gradually lose their sight for eternity.
The prophecies of the Elder Scrolls and the Heroes are interdependent, one cannot exist without the other.
http://www.uesp.net/wiki/General:Elder_Scrolls