In Shakespeare's “Sonnet 130,” the speaker takes a roundabout approach to describing his beloved. He uses every cliché he can think of but says that his mistress is none of them. Her eyes are “nothing like the sun.” Her lips aren't nearly as red as coral. He breasts are not white like snow but rather dun. Her hair is like “black wires.” Her cheeks lack roses. Her breath is hardly perfume; in fact, it rather reeks. Music is much more pleasing than her voice. Apparently, she even walks with a heavy step, not at all like a goddess.

The speaker is simply being honest, but we hope that his beloved has a sense of humor, for this description seems more like an insult than a compliment. Of course, we know, too, that the speaker is being realistic. Those old clichés are worn out and not overly descriptive of anyone anyway. Humans are imperfect, even messy, and the speaker's beloved is too. She has her faults, and the speaker is well aware of them.

Yet the speaker loves his mistress anyway. “And yet, by heaven, I think my love as rare / As any she belied with false compare,” he asserts at the end of the sonnet. The speaker doesn't need to use tired old metaphors to express his love. He loves her better by speaking the truth and appreciating what makes her distinct, not by likening her to unrealistic and widely-held ideals. She is simply human, and he loves her as she is.

In "Sonnet 130," Shakespeare's speaker suggests that the lady he loves is special because she is unique. She does not conform to the clichéd, stereotypical ideas about what beauty is or should be; instead, she is beautiful on her own terms.

For most of the sonnet, Shakespeare lists the contemporary, clichéd stereotypes of beauty. One of these clichés is that a beautiful lady's eyes should be "like the sun," and another is that her skin should be as white as "snow." A third cliché is that a beautiful lady's cheeks should be red like "roses," and a fourth is that a lady's voice should be like "music."

Shakespeare's speaker acknowledges that the lady he loves does not adhere to any of these clichés. At the end of the poem, he rejoices that this is so. He proclaims, "by heaven, I think my love as rare / As any she belied with false compare." The implication here is that he loves the lady, despite—and even because of—her divergence from the hollow clichés of ideal beauty, which the speaker refers to as "false compare." Indeed, that she has distinguished herself from common notions of beauty is a strength. Just as the value of a gemstone might be higher if it is particularly rare, so the lady in this poem is especially valuable and precious because she is rare.

In "Sonnet 130," Shakespeare describes the woman he loves as a real person instead of exaggerating her beauty. 

At first, his description seems almost insulting. He says that her eyes are dull -- not bright like the sun. Her lips are more pale than coral. Her hair is black wires. There are no roses in her cheeks. Her breath smells. Though he likes to "hear her speak," he knows her voice isn't as beautiful as music. 

He says, "I grant I never saw a goddess go; My mistress when she walks treads on the ground." Shakespeare is saying that she's not a goddess. She's a mortal woman and he recognizes her flaws and shortcomings.

In the end, Shakespeare makes it clear that loving someone when you see their flaws is what matters. He says that it's rarer to love someone with full knowledge of their faults than to idealize someone and love them for what they aren't. His love is the truer love because it's the one that's real. 

This sonnet is very different than many other love poems. Instead of idealizing his love, he humanizes her. 

The best and most simplistic answer to this question is: honestly.  Shakespeare is honest about his love, and describes her as an average person.  He says here eyes are "not like the sun" - meaning they are not overly bright.  He says that her is black - "black wires grow on her head".  He says that her breasts "are dun", meaning that they are not pure white but a more faded and average skin tone.  In Shakespeare time, pale white skin was the most attractive; browner skin suggested a hard-working or outdoor life.

Going on, Shakespeare explains that her breath is not like perfume, and her voice is not like music.  Again, he is being honest.  He is saying that she is not perfect.  Shakespeare does this in reaction to so many love poets that over-glorify their lady loves.  The metaphors used by the other poets suggest that the woman are like goddesses and are meant to be worshipped.  Shakespeare refuses to make that comparison here.  He actually refers specifically to that:

I grant I never saw a goddess go

My mistress when she walks treads on the ground.

However, his argument is that his love is even more pure than the love of all those flowery poets, because he is able to admit the flaws and limitations of his mistress and love her all the same.

And yet, by heaven, I think my love as rare

As any she belied with false compare.

He is implying that other poet's compare falsely, and so their love is not as rare or as pure.  His - being based in reality - is the true love.