Sheakespear, in this sonnet, used the Elisabethian model of sonnet, which is made of 3 quatrains and 1 rhyme couplet. 

The speaking voice wants to explain how he feels towards a person who he cheerish a lot. 

The whole sonnet is a celebration of his lover, which as he said in th 9th line "but the eternal summer shall not fade" he means that the beautiness of he or she won't ever fade,  while in the previous lines he said that even if the seasons are beautiful they fade, but he/she beautiness will never. 

In the last two lines, we can obsverve his love and affection towards his lover, because he declears that his loved one is the muse of his sonnets, as far as humans will breath he will always love her/him, and that he will write down this sonnets for him/her.