Okay, so, the novel is about a kid named Huck Finn having some adventures. Pretty clear.
But we think there's actually something more going on here. "Adventures" sounds like kid stuff. In fact, it sounds a lot like The Adventures of Tom Sawyer, Twain's earlier novel. This confuses people. Imagine if J.K. Rowling published Harry Potter and the Sorcerer's Stone and followed it up with Hermione and the Magic Cat, which wasn't so much about a magic cat as about immigration controversies in the U.S. In fact, Adventures of Huckleberry Finn, while it does have some adventures in it, is more about race and slavery in antebellum South.
So why mess with us this way? Part of the novel's charm is that it tricks us into examining weighty issues through the eyes of a naïve thirteen-year-old boy. Now, just picture Twain saying "Gotcha!"