Final answer:
The line from Shakespeare's Sonnet 18 speaks to the passage of time and the changing nature of the sun's brilliance, serving as a metaphor for the fleeting beauty of a person.
Step-by-step explanation:
In Shakespeare's Sonnet 18, the line "And often is his gold complexion dimm'd" refers to the changing aspects of the sun's appearance as well as the broader theme of the transient nature of beauty and life. This line suggests the passage of time and the impact it has on the brilliance of the sun, which can be seen as a metaphor for the fading beauty of a person. In the context of the poem, which addresses the enduring nature of the speaker's verse compared to the temporal beauty of a summer's day, this line underscores the transient quality of even the sun's golden complexion, hence encapsulating the notion that all beautiful things, like a summer's day or a person's beauty, will eventually fade.