Your conclusion must answer the question posed in your introduction. It should contain no new material and be relatively short when compared to the rest of the essay. Essentially, it reiterates the main argument you have proposed.
Your conclusion is your last opportunity to remind the reader what your essay set out to do, and how you accomplished it. Your conclusion should, therefore, revisit the question posed, and, briefly, summarise how you answered it. Whereas your introduction will set out what you proposed to do or show, your conclusion will talk of what you did achieve through the course of your essay.
Example of a good conclusion