The inscription concerns the priests and the King of Egypt. It was a decree passed by a general council of priests on the first anniversary of the coronation of Ptolemy V Epiphanes - the 13 year old King of Egypt. The text concerns the honours bestowed on the king by the temples of Egypt in return for services rendered by him, both at home and abroad. Priestly privileges, particularly those of an economic nature, are listed in detail.
See more about the Rosetta Stone here.