A good thesis narrows your topic to an idea that you can successfully develop within the framework of your essay. A thesis can be clearly defined and still lead to a rambling essay if it is not adequately focused.

Too broad: "The average American is exposed to many health hazards."

This thesis, though clearly defined, is so broad that you would never be able to cover it adequately in a short essay. You would wind up either jumping from one health hazard to another, discussing each only superficially, or zeroing in on one or two health hazards and, thus, failing to demonstrate your own thesis.

Better example: "The Constitution of the United States should be amended to prohibit the production and sale of cigarettes."

This thesis commits you to an idea that you can carefully analyze and defend in four or five pages.