You just hit the highlights or key takeaways either of what you've found in your research or what you plan to research going forward. It's not intended to be substantive.
For instance (and don't use this because I have no idea if it's true, I'm just making it up for style...):
This essay will compare & contrast ancient Mesopotamian and Greek cultures & lifestyles. I will demonstrate that Mesopotamia was a matriarchal society dominated by women elders who allocated jobs, and therefore determined the socio-economic hierarchy of their communities. With women in the leadership roles, ancient Mesopotamia was also a very humanistic and cultured society - highly focused on the arts and community collaboration. The ancient Greeks, on the other hand, were patriarchal. Success in that society was determined by competition, a passion to conquer others at all costs, and the ruthless cultivation of power relationships. Ancient Mesopotamians ultimately achieved more than the Greeks because of their approaches to community, the arts and their enlightened distribution of power & wealth.
You get the idea. If you're just starting, it's essentially a distillation of the outline for your paper and what you will research. It is always a good idea to prep an outline of the topics you will cover and the order (or flow) of how you'll cover them. It keeps your writing and thought/logic process organized. And makes your final result more impactful for the reader. If you are being asked to write a single sentence, start with something that contains about as much as what I've used (or whatever you can come up with), then keep cutting it down to just the essence (e.g., The ancient Mesopotamians were enlightened and collaborative and therefore flourished, while the Greeks were competitive and power-hungry and fell short of the successes of their neighbors to the North).
Good luck!
