The first sentence introduces Huck in a colloquial, friendly manner: "You don't know about me." From the very first words of the novel, Twain makes it clear that Huck is the narrator, and that the reader will hear the story of his adventures directly from him. In addition, to make it clear to readers unfamiliar with The Adventures of Tom Sawyer that this novel exists independently, Huck explains that if they haven't read Twain's earlier work, it "ain't no matter."
The bold letters or phrases emphasize the fact that Huckleberry Finn's story began in the earlier novel.