Sonnet 18 is a poem written by Shakespeare, and it’s a love-related poem. The general image of the poem is pretty sweet. The writer is comparing his love with summer’s day. He uses a lot of metaphors and poetic expressions, mostly to describe his lover.
And every fair from fair sometime declines,
By chance, or nature’s changing course untrimmed:
But thy eternal summer shall not fade
This is one of the expressions and metaphors he used. These words mean that every beauty in the world sometimes fades, but his lovers will not fade forever. We can say summer is a metaphor representing beauty.
Shakespeare wrote this poem for his lover and to make other people remember her. I can conclude this by reading this part:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.