Tone refers to the author's attitude toward a subject or an audience. It can be stablished by the words chosen, or the point of view of the author of a subject. Every argument has a main idea that is being addressed, by approaching to it in a specific manner the author sets the tone. It can be formal, informal, serious, sad, sarcastic, etc. It's correct use can affect how the audicence feels and how they take it. If you are talking about war in Iraq, for example, you would probably use a serious tone, if not the audience can get mad or not take you seriously.