The poem begins by posing a question about the superiority of summer to winter and proceeds to extol the virtues of the former. Shakespeare employs various metaphors, such as comparing the beloved's beauty to a summer's day, to make his point. However, he acknowledges that even the most magnificent and extraordinary aspects of nature, including summer, are susceptible to the passage of time and may not endure forever.
The couplet serves to provide a resolution to these concerns. While physical beauty may be ephemeral, subject to the changes of time and the ravages of age, the poet asserts that true beauty possesses a timeless quality and has the potential to endure eternally in the form of poetry.
Shakespeare proposes that genuine beauty transcends its physical manifestation and persists in the form of poetic words, which are capable of capturing and conveying its essence even as time passes. The couplet emphasizes the power of poetry as a medium capable of preserving and disseminating beauty beyond its temporal existence.
In conclusion, the couplet brings closure to the poem's ideas by demonstrating that while ephemeral physical beauty may ultimately fade, its significance and loveliness can be preserved in the everlasting lines of a poetic work. The poem's main point, which emphasizes the enduring nature of true beauty, is powerfully reinforced through this conclusive couplet.