A thesis statement can be many things. For advanced writers, there's even such a thing as an implied thesis statement.
But if you're asking this question, I assume you want to know the basic definition/usage: A thesis statement gives the central point of the essay/article as well as the main points you will be discussing.
For the basic 5-paragraph essay (which is a good model for inexperienced writers and people taking a writing test) the thesis statement will have the central position and three points (each of which will be developed in the three body paragraphs)
"Writing Hubpages is a good idea because it helps you make connections with fellow writers, it increases your exposure, and it helps you to create backlinks for your website."
"Writing Hubpages is a good idea" is the point of your whole essay.
"it helps you make connections with fellow writers" will be the focus of body paragraph one.
"exposure" will be the focus of body paragraph two.
"backlinks" will be the focus of body paragraph three.
A solid thesis statement not only helps your reader know what he or she will be getting into, it also helps you, the writer, structure your writing better.