In an essay, that one main thought has a name: the thesis statement. That thesis statement is supported by a number of paragraphs, each of which covers its own little piece of the main thought. Those "mini main thoughts" are called topic sentences. The rest of each paragraph is comprised of evidence, analysis, interpretation, and explanation - each as required to prove the main thought.
When all of those elements, the thesis, topic sentences, and support, combine to seamlessly advocate one position about one topic, the essay has the quality of unity. When any one of those elements is a bit wonky-skewed or off-topic, the writing must be revised to get it back on track. The most common error new writers make is to forget to unify evidence in a paragraph with that paragraph's topic sentence.
Aligning Topic Sentences and Evidence
Revising for Unity
A paragraph that lacks unity:
In order to revise this paragraph for unity, we have to first pick out the one idea we feel is the most important. It seems as though the main idea isn't really the first sentence of the paragraph because not all of the sentences in the paragraph are about how the uncle dresses. It seems all of the sentences are about the uncle wanting to be a cowboy in some way. Let's revise the topic sentence.
Instead of writing My uncle Fred dresses like an old-fashioned cowboy, let's emphasize the real main idea: My uncle Fred has always wanted to be a cowboy.
Now, if you were to say "My uncle Fred has always wanted to be a cowboy," to someone, that person's most likely response would be, "Why do you say that?" That means the rest of the paragraph must answer that question. Let's add the new topic sentence to the beginning of the paragraph and look carefully at the rest of the sentences and ideas to make sure they are clearly supporting the topic sentence.
A paragraph that demonstrates unity:
With a bit of revision, sentences that refer to the uncle's job as an accountant and his wife now more clearly support the main idea. As they were stated previously, they were off-topic, making the paragraph difficult to read.
When writing your essays, keep in mind the quality of unity, the idea that a piece of writing should express one clear position about one narrowed topic.