What is the intention of the poet here? What do the colossal wreck and sands symbolize?

Shelley's intention is to show that political power is never permanent and that even the greatest, most powerful ruler will not survive forever; thus Shelley seeks to satirize politicians, or dominant rulers, and call the reader's attention to the passing of political power. The poem has a weighty theme, so Shelley has used the formal sonnet form. 
The "colossal wreck" is literally the giant remains of the statue of the once-powerful ruler Ozymandias, with the "shattered visage" (face and expression) and the now broken "legs of stone" which have parted from the body so that they are "trunkless." The adjective "colossal" is also used ironically and metaphorically to refer to the enormous power this "king of kings" once had but has now lost, as Ozymandias has succumbed to death. Even his statue lies wrecked in the "lone and level" sands, which will endure far longer than he did! We notice the alliteration in this line, suggesting something long-lasting, even eternal. Therefore, the "sands" symbolize the impassive natural world, which is stronger than any political power or whim, which lasts for a comparatively brief time compared to the earth, which has witnessed man's changing ideas and histories for centuries.