I taught this excellent book for the first time in September of last year, and so I will offer you a basic framework that you can use to approach this novel and hopefully this will give you a bit more of an idea of what Twain was trying to communicate through it.
When we talk about what a book is about, we are referring to a concept known as theme in the study of literature. This means the universal message that the author is trying to communicate. When we think of this novel, there are arguably four main themes we can identify:
Coming of Age
This is a book about the moral development of Huck as a character. He is someone who obviously matures as a response to what he sees and witnesses, and his high point comes when he is willing (in his mind) to forsake heaven to try to rescue Jim and help him escape. He at this point is a character who has worked out who he is and how he stands in opposition to various social ills of the day.
The Hypocrisy of Christianity
Good Christian characters are shown to be weak, naive and ineffective. Consider the way that the Judge was unable to really protect or help Huck. Likewise, consider the way that Christian characters are shown to treat slaves terribly.
The Inhumanity of Slavery
Throughout the novel, we are presented with a view of slavery that is shocking and abhorrent. Jim is variously treated and viewed as an animal to be hunted down, a supply of cash, and a commodity to be sold at whim. It is only Huck who apparently comes to see him as a human.
Civilisation vs. Nature
The symbol of the river and the carefree life that Jim and Huck enjoy is contrasted strongly with the various intrusions that civilisation brings. These show the disorder of society and help us to understand Huck's extreme reluctance to be "civilised." It is only on the river when they are alone that Huck finds peace and lives in harmony with himself and nature.
I hope these hints will give you some ideas about what this novel is about. Please do persevere in reading and studying it. You might like to pick any one of these themes and consider how they are followed through the text as a whole. Enjoy!