thesis statements

In my English class we have to come up with a thesis statement for our myths we are writing. What I am wondering is what is a thesis statement. Both of the people who were sitting besides me in my English class were asking me what it is and amazingly enough I did not know. I had written one many many times and yet I still did not even know what it was. How come lots of people write stuff but do not even know what there writing means. Its just like people who talk but don't even know what they are saying they were just brought up to say it. Do we call those people hypocrites or is there another name for it . . . .