The poem begins with the narrator meeting an anonymous man who has seen some ruins in the desert of an "antique land," and tells the narrator that he's seen two huge legs, without a body attached, standing by themselves in the desert.
Near the legs lies a "shattered visage," that is, the broken face of the statue, but there is enough of the face to see that the person who was the model for this sculpture had "a wrinkled lip, and sneer of cold command"--in other words, we are meant to understand that this broken statue is of a man who once was obviously a powerful ruler with not just a commanding expression but an unfeeling expression.
An important detail in the narrator's tale is that the sculptor knew his subject well and was able to capture the essence of the once powerful king.
The last five lines are the heart of this poem. On the pedestal is the King's boast to the world--"Look on my works, ye Mighty, and despair"--and this is the point at which the irony of the poem becomes clear. The traveller then explains that the "colossal wreck" of the statue is sitting in the middle of the desert, with nothing but sand in all directions.
The irony of Ozymandias' boast is that pride, vanity, strength, and the illusion of permanence are transitory and ephemeral--these things simply disappear into the sand, and what remains are just fragments of a once powerful king.