Moon letters were special Dwarven runes written in Ithildin that could only be revealed by the Moon at a certain time. 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 - they 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.
An example of these moon-letters would be on the map of the Lonely Mountain (Erebor) and surrounding areas that Gandalf gave to Thorin Oakenshield. The moon letters were the more cunning type mentioned above, and they were written on a midsummer's eve in a crescent moon. They said: