Not too fast, please.
The Rosetta stone is a huge slab of stone which was part of the door of a temple.
It was found in Rosetta, a city in the North of Egypt, on the Nile river, in 1799 by Napoleon’s army during the campaign of Egypt.
After the French army was defeated in 1801, the stone was given to the British, that’s why it is now in the British Museum.
On the rosetta stone, the same text is written in 3 languages : hieroglyphs, Demotic and ancient Greek. French archeologist JF Champollion studied these texts and he could decipher the Hieroglyphs thanks to the Rosetta Stone.