Moon letters are special runes written in Ithildin with silver pens. They were created by the Dwarves in a cunning fashion as they can only be seen by the moon of the shape and season on the day they were written. They are only mentioned in The Hobbit when Elrond held up Thror's Map to read to Thorin and Company.
Moon-letters were a system of writing invented by Dwarves. Moon-letters were ordinary Runes, but written in such a way that they could only be read when the light of the moon shone behind the writing. Some moon-letters could be read by any moonlight, but it was possible to make the writing even more secret. More sophisticated moon-letters could only be read by the same shape of moon, or at the same time of year, as when they were written.
Moon-letters were written with silver pens. The substance Ithildin made the from the precious metal Mithril, reflected only starlight and moonlight, and it is that Ithildin was used by the Dwarves to write their moon-letters.
An example of these moon-letters would be on the map of the Lonely Mountain (Erebor) and surrounding areas that Gandalf gave Thorin Oakenshield. The moon letters were the more cunning type mentioned above, and they were written on a midsummers eve in a crescent moon. They said: