Question

What is does it mean when it says to use thesis to introduce the main idea

Answer 1

The way a short paper is usually organized is in this manner (five paragraphs):
The first paragraph will start with an attention grabber, list the main topics of the body (next three) paragraphs, and then list the thesis sentence (the sentence that encompasses the three main points of the paper).
The second to fourth paragraphs should be the body paragraphs with topic sentences in each.
The last, or conclusionary, paragraph restates the thesis (paraphrasing is good), restates the three main points, and ends with a clincher sentence (like an attention grabber, but leaves the reader thinking about the paper).

Answer 2

The thesis statement is the point of your paper or the crux of your argument. Examples:

Although dogs can be quite loyal, cats have calmer demeanors and like to cuddle. (This would be a thesis statement introducing my argument that cats are better than dogs.)

A seemingly broken VHS tape can be fixed with a screwdriver and some simple steps. (Thesis statement for a paper on how to fix a VHS tape.)