Answer:
The answer is in the next line. The inscription reads, "My name is Ozymandius, King of Kings: Look on my works, ye mighty, and despair!"
Yet in the next line we read that there is nothing to look at - everything that he boasted of is gone, and the "lone and level sands stretch far away." Earlier we even read that his own visage (his face, as in, that part of the statue) is lying shattered on the ground.
The poem is about temporal greatness. Many people, especially egotistical rulers, think that what they have achieved will last forever. It never does.