In this poem, poetery is given a very high status. Its status (or at least the status of Shakespeare's own poetry) is almost that of a god because it can make someone immortal.
In the poem, Shakespeare is saying that his love is so much better than a summer day. One of the ways in which this is true is that his love will live forever while a summer day does not last very long (hath all too short a lease). The reason why his love will live forever is because he has written this poem about her.
You have got some beautiful answers already, and in very brief, I'll dare to add a reply with due respect to the above answerers.
In his sonnet 18, Shakespeare has taken the magnitude of poetry to such a stature where poetry is compared with a key to immortality. Poetry has a quality which can transform impermanence to permanence. That is what Sonnet 18 is about. The speaker, here, talks about transforming his love-affair into a poem and immortalizing their love.
Poetry is equal to eternal life. About this sonnet there is little that one can say except that it is one of the foremost poems in the language. The last two lines climax the poem’s argument. Here the speaker asserts that the poem will endure as long as human life endures, and that as long as people can see (and read), the sonnet will give life to the woman the speaker is addressing. It is the immortality of art that the speaker exalts over life’s transience.
The development in lines 1–8 asserts that natural beauty fades and dies. This assertion is essential to the point of the last six lines—that only art can give immortality.
At line 9, the speaker begins asserting that, unlike natural objects, the lady’s summer (i.e., the beauty of her disposition) cannot fade, and that even Death (line 11) cannot claim her as long as she is the subject of the "lines" of the poem. Here the speaker is complimenting the power of his own poetry as well as the beauty of the lady.
The last two lines are related to the previous twelve because they all concern the value of the lady and the nature of time. "This" refers to the sonnet itself, and the giving of life is the immortality that is conferred by subsequent generations of readers, who will always remember the sonnet and the lady and therefore create a continuous collective mind of memory.