Sonnet 130 makes fun of the hyperbolicExaggeration for a particular effect. love poetry of the time.
The speaker is describing his "mistress". This could mean his wife, his lover or simply a woman he admired.
Instead of over the top praise, he seems to be describing her as being not very attractive. He points out that he is just being realistic, but he ends by saying she is just as pretty as any woman who is "belied" (lied about) with false comparisons.