Short answers, pulled from the story.
The Rosetta Stone is a granodiorite stele inscribed in 196 BC with the same decree in three scripts: Ancient Egyptian hieroglyphs, Demotic, and Ancient Greek. Its importance lies in providing the parallel texts that scholars needed to decipher Egyptian hieroglyphs, which had been unreadable since shortly before the fall of the Roman Empire.
Lieutenant Pierre-Francois Bouchard, a French army officer, spotted the Rosetta Stone in July 1799 while French soldiers were strengthening the defences of Fort Julien near the Egyptian port of Rosetta. He and his commanding officer, Colonel d'Hautpoul, immediately recognised its potential significance and informed General Menou.
Champollion used the Greek text as a key and identified phonetic characters in the hieroglyphic text by looking for cartouches containing Greek royal names. After comparing the Philae obelisk inscriptions naming Ptolemaios and Kleopatra, he constructed an alphabet of phonetic hieroglyphic characters and announced his findings on the 27th of September 1822 in a lecture to the Academie royale des Inscriptions et Belles-Lettres.
The Rosetta Stone has been on public display at the British Museum almost continuously since June 1802, making it the museum's most-visited single object. It was taken to Britain under the terms of the Capitulation of Alexandria in 1801 after the British defeated the French in Egypt, and King George III directed that it be placed in the British Museum.
The stone records the Memphis decree, issued on the 27th of March, 196 BC, by a congress of priests gathered at Memphis on behalf of the twelve-year-old King Ptolemy V Epiphanes. The decree established the divine cult of the new ruler, recorded his gifts of silver and grain to the temples, and directed that copies be placed in every temple inscribed in hieroglyphs, Demotic, and Greek.
Zahi Hawass, then Secretary-General of Egypt's Supreme Council of Antiquities, publicly called for the stone's repatriation in July 2003, describing it as the "icon of our Egyptian identity", and reiterated the demand in 2005, 2009, and 2022. The British Museum responded in 2005 by presenting Egypt with a full-sized fibreglass colour-matched replica, initially displayed in the Rashid National Museum in the town of Rosetta.