The poet addresses this sonnet to his beloved or friend. He says that marbled or gilded monuments built in memories of princes etc, shall not outlive this sonnet. His beloved shall shine more bright in this sonnet than these stoned monuments. These have been left uncared for and are now spoiled by uncleaned behaviour of time.
The poet says further that wasteful wars shall destroy these statues and monuments. Fights or wars shall ruin the work of masonry, that is, these structures. When this happens, this will not destroy this sonnet which records her memory. Even the sword of Mars , the god of war or wars ' fast moving fire shall not destro it.
The post 's beloved shall go ahead against death and all enmity which forgets things and seeks to destroy them. Her praise in this sonnet shall have a place even in posterity. It will survive this world even to the doomsday.
The poet says further that his beloved shall arise till the judgement day. She will live in this sonnet and shall also live in her lover 's eyes.
Hope this helps... :D