The Silver Tablet is an important item in Ultima VI. Engraved on the tablet are the detailed instructions on how to translate the Gargish Alphabet into English -- the only instructions ever known to exist at that point in history.
Zoltan was ordered by Captain Johne to deliver the tablet to Mariah. But the pirates under Captain Hawkins attacked Zoltan's gypsies and the tablet got broken in two. The pirates got away with the larger half and Zoltan kept only the smaller one.
The Avatar needed the tablet to translate the Book of Prophecies, but only half had found its way to the Lycaeum, and Mariah needed both halves. The Avatar had to find the complete Treasure Map of Hawkins to find the hidden treasure and with it the bigger half of the tablet.
With it in full, the Book could be translated by Mariah, revealing the prophecy of the False Prophet.