Vala had obtained an Ancient stone tablet, written in code, and the person she had stolen it from had indicated that it supposedly led to an incredible Ancient buried treasure somewhere on Earth. Although Vala had the cipher to translate the tablet, she needed Daniel's expertise to interpret it, and she brought the tablet to Earth, seeking Daniel's help.
Using Vala's cipher, Daniel translated the tablet and determined that it did indeed reference a treasure in an unspecified location, and that it had been written and signed by "Myrddin." The name Myrddin also appeared in a database brought back from Atlantis that included a log of the names of the Ancients who had left Atlantis while under siege from the Wraith. Because the Ancient name "Myrddin" translates into English as "Merlin," Daniel concluded that Merlin had been an Ancient who had returned to Earth from Atlantis thousands of years ago. Legend associates Merlin with the Vale of Avalon, known today as Glastonbury, and the tablet eventually led SG-1 to discover Merlin's Ancient treasure buried in the caves beneath Glastonbury Tor.
Cross Reference: Avalon, Glastonbury, Vala Mal Doran, Merlin, Merlin's Treasure
Episode Reference: Avalon