In this sonnet , Shelly illustrates the valley of human greatness and the failures of all attempts to immortalise human achivements .Ozymandias was a great Egyptian king.A life  - like statue of the king was made to immortalise him.But now the statue lies broken and disfigured in a desert.

A traveller from Egypt sees two huge and trunkless legs of a statue in the desert.Near them lies, half - buried , the broken face of statue. On this face one can see the expression of arrogance and a sense of authority which have been skilfully depicted by the sculptor. Int he pedestal the following words were inscribed : 
"My name is Ozymandias  and I am a Great King. Look at the great deeds that I have accomplished and which nobody can equal." The Broken statue is surrounded by a vast desert.

The poem reflects that human glory and pomp are not everlasting . Time works havoc with monuments and statues. Thus, the poet strikes a moral in a suggestive manner.

Source : It is my from a refresher guide.
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The speaker describes a meeting with someone who has traveled to a place where ancient civilizations once existed. We know from the title that he’s talking about Egypt. The traveler told the speaker a story about an old, fragmented statue in the middle of the desert. The statue is broken apart, but you can still make out the face of a person. The face looks stern and powerful, like a ruler. The sculptor did a good job at expressing the ruler’s personality. The ruler was a wicked guy, but he took care of his people.

On the pedestal near the face, the traveler reads an inscription in which the ruler Ozymandias tells anyone who might happen to pass by, basically, “Look around and see how awesome I am!” But there is no other evidence of his awesomeness in the vicinity of his giant, broken statue. There is just a lot of sand, as far as the eye can see. The traveler ends his story.