The first six lines deconstruct various metaphors that, even by Shakespeare’s time, had become poetic cliches.
The eyes of the speaker’s mistress do not emit blinding radiance, her cheeks are not literally the colour of roses, and her breasts look positively tan (considered ugly at the time, as exposure to the sun accompanied manual labour and low social status).
Her lips are not quite coral-bright, and her hair is nothing special (fair or golden hair would have been considered more beautiful than brunette at the time).
The opening line of this sonnet gave Anthony Burgess (of Clockwork Orange fame) the title of his fictional biography Nothing Like the Sun: A Story of Shakespeare’s Love Life.