The main metaphor in the poem is the comparison of the speaker’s love to a summer day.
In the first line, the speaker notes that he is going to compare someone to a “summer’s day.” This person, his lover, is quite a catch. In fact, she is lovelier than a summer’s day! She has other advantages.
But thy eternal summer shall not fade
Nor lose possession of that fair thou owest
Her eternal sunshine—a metaphor for her beauty—will last forever, unlike a summer’s day, because she is so lovely. Although everything that is fair has to decline, she will not. She will be beautiful “So long as men can breathe or eyes can see.”
In this famous sonnet, we see the traditional practice of comparing one’s girl to something in nature. Shakespeare often did this, and liked to make fun of it, but in this case we have a simple, straightforward metaphor that will really make a girl feel special.