This sonnet is one of Shakespeare's more famous poems because it plays off the traditional ideas of female beauty and turns those trite expressions on their head in order to make his ultimate point about how special his mistress is. It uses the traditional sonnet structure to illustrate his argument. The first three quatrains serve as the examples, and the final couplet draws the conclusion.
Traditional poetry of Shakespeare's age would have compared eyes to the radiance of the sun, lips to the color of red coral, skin to the white of snow, and cheeks as rosy red. The speaker of this poem uses all of those cliched ideas (stock metaphors), but with each one states that his mistress has NOTHING of that comparison to be true in a description of her. It would seem by reading only the first 12 lines of the sonnet that his lady is quite an unpleasant looking thing. The whole point of the sonnet becomes clear in the final couplet. It is here that he explains that " and yet, by heaven, I think my love as rare / As any she, belied with false compare." What this means is that his lady is more rare and therefore special than any "she" meaning any other woman. The speaker is actually paying the lady the highest compliments because he is not falsely comparing her to cliched ideas. He likes her for what she is and how she is. False flattery doesn't mean anything, and any man who says that a woman's eyes are bright as the sun is using an untrue hyperbole. Any man who says that his woman floats above the ground like a goddess is using an untrue hyperbole. He is saying that he isn't going to tell lies (use false compare) because he loves her for who she is.