What do the lone and level sands described in "Ozymandias" represent?

The lone and level sands represent or symbolize that nothing at all is left of Ozymandias's once-mighty kingdom except the broken statue of the tyrant.
In the inscription on the statue, a sculpture left strewn across the desert floor, Ozymandias assumes that his great kingdom and its grand works will be spread out all around his statue for any traveller to see. He expects any visitors to be very impressed with his power. He warns other rulers not to dare to confront him, implying that he is invincible. His statue says:

Look on my Works, ye Mighty, and despair!

This statement has become ironic. Rulers now looking on his kingdom, nothing more than a desert of sand, should despair not because they are afraid of him, but because they too will be reduced to nothing.