The poem itself will give life to the speaker's beloved. Though the beloved's looks will eventually fade, and he himself will one day die, he will nonetheless live on in some sense, immortalized by the beautiful words written about him by this great sonneteer and dramatist.
This is what Shakespeare means when he says that his beloved's "eternal summer shall not fade". Summer itself, no matter how beautiful it is, will eventually die out, giving way to the mists and mellow fruitfulness of autumn. That's why the answer to the speaker's opening question "Shall I compare thee to a summer’s day?" is a resounding no.
The speaker thinks about it for a while, but then realizes that it would be inappropriate to make such a comparison because summer, unlike his beloved's soul, is destined not to last. And besides, summer is not always temperate, that is to say not always characterized by mild temperatures. So this isn't a particularly flattering comparison on at least two different levels.
"Sonnet 18", as well as being one of the most famous love poems in the English language, is also a paean of praise to the immortalizing powers of art. In the words of the old Latin phrase Ars longa, vita brevis, which roughly translates as "Art is long, but life is short". In other words, art endures from one generation to the next, long after those who created it have passed away. In this way, the love that the speaker of "Sonnet 18" has for the object of his affection, is to be immortalized forever in verse.