Tone refers to the author's attitudes toward the characters and the story, and even toward the implied readers. This might be the most hazy area of literary criticism ever. For one thing, we can't read the author's mind. For another, we can't assume that the narrator's point of view reflects the attitude of the author. We have to make intuitive leaps. If we're lucky, we can find some juicy quotes from the author that help explain the tone, like this one from The Chocolate War's author, Robert Cormier:
The thing I'm trying to do is communicate with the reader – communicate the emotion I want him to feel. I sacrifice everything to that. I want to hit the reader with whatever emotion I want to portray, or whatever action that will make it vivid. […] I want the reader to feel the emotion of the characters. And I would use any word, any unpretty image, to communicate that emotion. (source)
This is not to say that there aren't other elements of Cormier's tone in this novel, but the dominant tone is emotional. As the quote stresses, he's trying hard to stir up our emotions, and let us experience a bit of what the different characters are feeling. Is he successful? Does The Chocolate War provoke strong emotions in you? If so, what are some of those emotions, and what actions or images trigger them? If not, why might this be? Do you want the stories you read to make you feel emotional? Why or why not?