The adventure of Huckleberry Finn is a novel written by Mark Twain. He is an anti-slavery person, so he writes about it.
Taking into account the theme of slavery, the author demonstrates slaves life through Jim. He wants the reader to feel sympathy and empathy for Jim who is one of the main characters.
Considering the idea of freedom/liberty, you can mention the theme of slavery and stand out that Jim runs away from his owner, Miss Watson (when he hears that she wants to sell him). So he decides to work to obtain money so he can buy his family's freedom.