The Lost city tablet was an ancient tablet written in Ancient that appeared in one of Daniel Jackson's dreams supplied by the Goa'uld Osiris. In this dream the tablet was found in Morocco and was over 10,000 years old.
It tells of a city inhabited by an advanced civilization and also contains an Address which was to trick Daniel Jackson into revealing the location of the Lost City which may have been in his mind from the time he was ascended. In this dream (which was set before he joined the Stargate program) he was able to read and understand Ancient. Luckily he did not know the symbols. (SG1: "Chimera")