Ozymandias is the Greek name of the Egyptian Pharaoh Ramses II, Rameses the Great. He ruled Egypt for 67 years and built many monuments in his own honor, the most familiar of which may be the temple of Abu Simbel.
In his sonnet, Shelley is reacting to those monuments, many of which have been damaged or destroyed and are mere fragments of what they once were. His message is that no matter how great we are--or think we are--fame and glory are only fleeting. This message applies to rulers, artists, and ordinary people alike.