Explain your interpretation.
"Shall I Compare Thee to a Summer's Day?" by William Shakespeare
This is a love poem but it is also about the power of poetry itself. The speaker compares his beloved to a summer’s day and makes the case that she is more fair, lovely and more timeless. It is this last quality from which he makes the transition of a poem about his love to a poem about the ability of his poetry to provide her with that timeless quality.
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
This sonnet will grant the immortality to the subject of his love. In line 12, he mentions “eternal lines” and this refers to the lines of the poem. So long lives this (the poem) and this (the poem) gives eternal life to thee. The overall idea is that artistic creation, particularly poetry, escapes the limitations of mortality. This is meant quite literally as something written down can certainly outlast someone's life. But it also abstract. A poem can become part of a literary history and culture. The very idea of a poem can become common knowledge. The memory is intact materially (on paper) and immaterially (the idea, thought and memory).
For example, Romeo and Juliet is a material thing: a written play. But it has become part of cultural knowledge. We know the story just by mentioning the title. The very idea of the story has outlasted Shakespeare.
Aside from its surface meaning as a love poem, sonnet 18 has three additional perspectives. The first, in the first and second quatrains, is a view of nature as a violent, degenerating source that erodes beauty. The second, in the third quatrain, is the idea of love that can defy the "shade" of "Death" as the beloved shall "growest" in something bigger and longer lasting than they: "eternal lines to time." The third perspective, presented in the couplet, is a definitive statement of poetry as an immortal power, as something that transcends time, and as something that has the power to bring someone or something else along into immortality with it: the subject may join in poetry's eternal immortal power.
I completely agree with the posts above and would add that while Shakespeare seems rather confident about the enduring quality of his poetry, he isn't the only well-known poet to state this same theme. Edmund Spenser, who is a contempory poet to Shakespeare wrote a sonnet (Sonnet 75) in which he claims that while everything of world will die in dust, "My verse your virtues rare shall eternize . . . (and) Our love shall live [in the words of the poem] and later life renew." I am sure with some sleuthing you could uncover several poems that share this theme -- I vaguely recall an Emily Dickinson poem with this same assurance. Does it suggest something about the nature of poets?
I agree with Post #2 except that I think it's not about the power of poetry in general -- this is Shakespeare's love poem to his own poetry.
Look at the lines quoted above. Shakespeare is saying that his poem is going to live as long as men breathe. It's a really arrogant statement, isn't it (of course, he's right so far, but that's another issue...). So basically, Shakespeare is saying that he is such a great poet that his work will live forever.
So I agree in general with Post #2, but I think that Shakespeare is extolling his own poetry as much as he is poetry in general.