In "Sonnet 18" the speaker is comparing a person to a summer's day and stating that this person is MORE lovely and MORE temperate. That by itself is rather surprising because we generally think of summer as the most lovely of seasons, but the speaker goes on to explain. He reminds us that summer sometimes has rough winds, cloudy skies, and is too hot. He is building to his ultimate point in lines 7 and 8. Here he says
And every fair from fair sometimes declines,
By chance or nature's changing course untrimmed.
What he is foreshadowing is that all things, especially beautiful things eventually fade away. People age and seasons change. This point sets up the rest of the sonnet. Lines 9-12 speak specifically about the person the speaker is addressing. He says the beauty of this person will never fade and that death will not be able to brag about him or her because this poem ("eternal lines to time") will live on and in it, the beauty of the person will live on so long as the poem exists. As with most sonnets, the ultimate point of the poem comes in the last lines. In this case, it would seem the the speaker is right. Here we are more than 400 years after this sonnet was written thinking about the beauty of the person being discussed. We are doing that because the "lines" have lasted that long.