Game Engines

Anyone have ANY idea how these work? Currently trying to write an essay on how I would make my game concept real using a Game Engine like Unreal or Unity or something.
It's actually pretty difficult I've got a book called Mastering Unreal Technology: The Art of Level Design, although it doesn't seem to help that much. Although I will figure out a way to write this essay. Wish me luck guys.
Also I still need to write the other half of my Game Concept. Should have it done by Friday!
Wish me luck!

Thanks.
Chris.