The speaker in this poem is talking to a star. Weird, huh? Well, in poetry, you can get away with anything. So what does he tell the star? Well, he starts off by saying how he wishes he were as "stedfast" as it is. Because the star he's talking about doesn't move, it's likely that Keats means the North Star (a.k.a. Polaris). The North Star, of course, is the one star that doesn't move in the sky, because it is directly above the North Pole. Thus, sailors use it as a point of navigation.
All very interesting, but why is Keats's speaker talking to the star? Hard to say, because, then in the next line, he shifts gears, and starts talking about all the ways in which he doesn't want to be like the star. Now it seems he doesn't like the idea of spending all eternity in loneliness, watching the chill-inducing spectacle of water flowing endlessly around the earth, and snow falling on barren landscapes. Hm.
So what was up with all that wanting to be a star business? In the ninth line, we start to get a hint. The speaker wants to be like a star in the sense that the star doesn't move, and never changes. But he wants to take that whole never moving, never changing bit, and put it in a different context. He wants to spend all eternity with his head lying on his girlfriend's breast. And if he can't spend all eternity like that, he'd rather die, by swooning. So, basically, he'd like to be like the star, but...