The speaker begins the sonnet by declaring that the eyes of the woman he loves are "nothing like the sun." He also says that "coral is far more red than her lips' red." Ostensibly it may seem here that the speaker is criticizing his lover for not having eyes as bright as the sun or lips as red as coral. However, the speaker is really making the point that these parameters by which beauty is conventionally judged are false and insincere. The implication is that, by contrast, his lover's beauty is more authentic and more individual.
The speaker continues to expose more cliched ideas about love, and it is significant that most of these ideas pertain to a woman's physical appearance. The speaker says that there are no "roses ... in her cheeks" and that her breasts are not white like snow but rather "dun," meaning a dull grey or brown color. By indicating that his lover does not have the physical attributes commonly believed to be synonymous with beauty, the speaker is indicating that his love is sincere because it is based on more than just the woman's physical attributes.
At the end of the poem, the speaker asserts that his love is "as rare / As any she belied with false compare." In other words, the speaker's love is not of the cliched, common variety; his love exists independently of the standards that society sets and is thus more sincere than love which measures itself against those standards. The speaker does not falsely compare his lover to far-fetched cliched ideals but, rather, he loves her because she is unique and incomparable.