The Rosetta Stone was an object of unknown properties stowed in Gotham's Natural History Museum. J'onn J'onzz intercepted information about Intergang being after it, so he deployed Wonder Woman and Batman to thwart them. However, instead of Intergang, it was Circe who attempted to break into the Museum. The reason behind Intergang's interest in the artifact remains unknown.
The Rosetta stone is a real life object, a stone slab with an inscription in Egyptian, Demotic and Greek. It enabled linguists to decipher hieroglyphs, and the term "Rosetta Stone" has since taken the figurative meaning of a key to understanding something.