The function of the expository essay is to explain, or to acquaint your reader with a body of knowledge. By explaining a topic to the reader, you are demonstrating your own knowledge.
For example, if you are asked to write an essay about taming dragons, you decide what you plan to concentrate on, create a paragraph structure, and describe the process step by step. An essay becomes more complicated when a position has to be defended, as in a persuasive essay.