Ozymandias thought so highly of himself that he had this statue built with that inscription to intimidate anyone who might come near it. He meant for it to be a warning to not try and invade or attack, the despair part is meant to be the person despairing because they could never be as great as Ozymandias.
The meaning is twisted though, as the statue crumbles with time showing that all leaders -- especially those who value themselves so highly--will fall.
