The inscription on the Rosetta Stone is a decree passed by a council of priests. It is one of a series that affirm the royal cult of the 13-year-old Ptolemy V on the first anniversary of his coronation.  

In previous years the family of the Ptolemies had lost control of certain parts of the country. It had taken their armies some time to put down opposition in the Delta, and parts of southern Upper Egypt, particularly Thebes, were not yet back under the government's control.

Before the Ptolemaic era (that is before about 332 BC), decrees in hieroglyphs such as this were usually set up by the king. It shows how much things had changed from Pharaonic times that the priests, the only people who had kept the knowledge of writing hieroglyphs, were now issuing such decrees. The list of good deeds done by the king for the temples hints at the way in which the support of the priests was ensured.    

The decree is inscribed on the stone three times. In hieroglyphic, suitable for a priestly decree.

Egyptian script used for non-religious texts.  

The language of the administration. 
The importance of this to Egyptology is immense.  

Soon after the end of the fourth century AD, when hieroglyphs had gone out of use, the knowledge of how to read and write them disappeared. 

In the early years of the nineteenth century, some 1400 years later, scholars were able to use the Greek inscription on this stone as the key to decipher them.  

Thomas Young, an English physicist, was the first to show that some of the hieroglyphs on the Rosetta Stone wrote the sounds of a royal name, that of Ptolemy. The French scholar Jean-François Champollion then realized that hieroglyphs recorded the sound of the Egyptian language and laid the foundations of our knowledge of ancient Egyptian language and culture.  

Soldiers in Napoleon's army discovered the Rosetta Stone in 1799 while digging the foundations of an addition to a fort near the town of el-Rashid (Rosetta). On Napoleon's defeat, the stone became the property of the British under the terms of the Treaty of Alexandria (1801) along with other antiquities that the French had found.  

The Rosetta Stone has been exhibited in the British Museum since 1802, with only one break. Towards the end of the First World War, in 1917, when the Museum was concerned about heavy bombing in London, they moved it to safety along with other, portable, 'important' objects.