The quote you have identified comes from the first chapter and involves Huck's feeligns about living with the Widow Douglas and Miss Watson and their attempts to "sivlize" Huck, which he does not take kindly to. Note what the whole paragraph says and how it refers to Christianity and Huck's thoughts about it:
Now she had got a start, and she went on and told me all about the good place. She said all a body would have to do there was to go around all day long with a harp and sing, forever and ever. So I didn't think much of it. But I never said so. I asked her is she reckoned Tom Sawyer would go there, and she said not by a considerable sight. I was glad about that, because I wanted him and me to be together.
Note how this presents Christianity. In the mind of Miss Watson, heaven is presented as a place where you have "a harp and sing, forver and ever." To Huck's mind, this is obviously an incredibly boring presentation of heaven, and one that is not attractive. In addition, his friendship with Tom Sawyer and Miss Watson's assurance that Tom will definitely be not going to heaven causes him to choose to go to hell to be with his friend because he wanted "him and me to be together." This begins a theme that runs through the entire novel concerning the hypocrisy of Christianity, when adult characters present an image of Christianity that is not exactly flattering, and indeed is contradictory. These contradictions are not lost on Huck, who chooses to reject the Christianity that is represented by such hypocrisy.
This quote begins and ends the sixth paragraph of Chapter 1 in which the Widow Douglas preaches to Huck Finn after supper, telling him about Moses. But, when Huck finds out that Moses is from ancient history, he loses interest in him. As he fidgets, the "tolerable slim" sister of the widow, Miss Watson, corrects Hucks bad manners. In her effort to make Huck behave, she tells him about the "bad place," and Huck, in his bored frustration, declares that he would like to go there. This remark, of course, upsets Miss Watson, who tells Huck it is wicked to say such things.
Now she had got a start, and she went on and told me all about the good place.
Miss Watson describes heaven as she perceives it; there she will walk around all day playing a harp. Again, Huck is not impressed; he asks her if Tom Sawyer might go there, and she replies "not by a considerable sight." Then, Huck becomes glad to think this because then he and his friend Tom will get to be together.
This passage is just one of many illustrations of the religious hypocrisy of Miss Watson who believes herself so righteous when she owns the slave Jim and is hypocritical in other ways. Here, too, Twain's satiric humor comes into play as the unmannerly and tobacco-smoking Huck is actually a much more guileless person than the widow and her sister.