Imagine that the omniscient, omnipotent creator of the heavens and earth could be an advanced version of a guy who spends his weekends building model railroads or overseeing video-game worlds like the Sims.
If you accept a pretty reasonable assumption of Dr. Bostrom's, it is almost a mathematical certainty that we are living in someone else's computer simulation.
This simulation would be similar to the one in "The Matrix," in which most humans don't realize that their lives and their world are just illusions created in their brains while their bodies are suspended in vats of liquid. But in Dr. Bostrom's notion of reality, you wouldn't even have a body made of flesh. Your brain would exist only as a network of computer circuits.