You should go through Ozymandias line by line, however summary is provided below;
Ozymandias is a sonnet written Shelley. The poet met a traveler who once was in Greece and he tells the story of a mighty king. The king thought that he would remain forever but with time everything ruined. The powerful kingdom and everything related was destroyed and decade. It was the interpretation of the traveler from what he saw there. The poet conveys that everyone is mortal, and as the time passes, changes are bound to take place.