It was a partially damaged stone tablet which was excavated in Rosetta (Egypt) in about 1800. It had text written on it in three different languages - Greek, Demotic and Heiroglyphic.
As ancient Greek was a known and understood language, the other two languages could then be deciphered and it allowed historians to decipher writings of ancient Egypt for the first time
As an aside, It is a commonly used term, referring to anything which is used as a key for deciphering the previously undecipherable.