Here we are introduced for the first time to the main character in this poem, the speaker's "mistress."
Today, when we use the word "mistress," it's usually to refer to a woman who is dating a married man. In Shakespeare, though, it was more general, like "my love" or "my darling."
The speaker jumps right into his anti-love poem, letting us know that this lady's eyes aren't like the sun. Well, so what? We wouldn't really expect them to be, would we?
As we read the next few lines though, we see that the comparison is a standard way of praising a beautiful woman in a poem. It's like saying, "her eyes are like sapphires."
Our speaker is refusing to fall back on clichés though, instead telling us that this simile doesn't apply at all.