The main idea in most of Shakespeare's sonnets is presented by the final two lines, the rhyming couplet. Many sonnets take love as its subject and use hyperbole or metaphors that compare a woman's beauty to objects in unrealistic ways. Here, the speaker refuses to do that. Rather than say that his lover's eyes shine like the sun, he says, "My mistress' eyes are nothing like the sun" (line 1). Her lips are not red as coral, her skin is not white as snow. She does not have roses in her cheeks or heavenly breath. Although he "love[s] to hear her speak," he knows "That music hath a far more pleasing sound" (lines 9, 10). He cannot compare her, faithfully, to a goddess; she does not float. But in the final lines, the speaker says,
And yet, by heaven, I think my love as rare
As any she belied with false compare.
In other words, he says, his love is special and unique, and he does not need to make false, flattering comparisons in order to retain her love or prove that his love is strong. The larger implication, I think, is that no real love needs to boast of itself in this way.