The main idea, the reason you're writing, the backbone of the essay. The aspect of text that conveys a writer's attitude toward the subject or the audience. Tone can be ironic, sarcastic, nostalgic, informative, plaintive, and so on. Unity in a paragraph implies a sustained purpose, and forbids digressions and irrelevant matter. The way elements like word choice and tone add up to create an effect on the reader.
Different voices create different effects on the reader.
An angry voice might make the reader defensive. A sad voice might make the reader feel sad, too. Or, a sad voice might make the reader sneer with contempt.