Like all of us in the morning, John Donne starts out angry at the sun. He feels way too good to be bothered by its shine and tells it to get lost and go bother other, lesser people. He tells the sun that love isn't some slave to the sun's movements or the changing of the seasons, so shove off, thank you very much.
Then he gets really cocky. He says he's better than the sun, stronger than the sun, even, because he can just shut his eyes and make that bright star disappear entirely. In fact, he's such a big deal that he tells the sun to look around and discover that the whole world resides right there in Donne's bed.
He extends this idea, telling the sun that he and his lady are all the countries and kings in the world combined, that everything else in the world is just pretending to be them. He ends by pitying the poor old sun and telling it that its job just got easier. Because the whole world now consists of just this one room, the sun doesn't have to travel—it only needs to shine on this couple and everything is bright and peachy.