In this sonnet "Shall I compare thee to a summer's day" Shakespeare compares an unnamed woman to a beautiful summer day. The woman comes out favorably in the comparison because sometimes the sun is too hot, or the wind blows too hard, and of course, summer ends. He says the woman's beauty will never end when he writes, "but thy eternal summer shall not fade" and that death can not "brag" that it has conquered her. Why? Because she "lives" inside the poem. When Shakespeare says the woman will "grow" within the "eternal lines to time" he means that people will remember her because they remember the poem. He closes with "So long as men can breathe or eyes can see/ so long lives this [the poem] and this gives life to thee."
Shakespeare seems to have had a lot of confidence about this poem living on and, as it turns out, he was right. Here we are discussing it many hundreds of years later.