To imagine this, just imagine creating one yourself, like a video game. You create a virtual world, you design a planet, even a universe if you choose to. You focus on one planet and design it, create life on it. You need to make the world self sufficient so you design the plants, animal life, etc, cycles, water, everything all around that. Then you design humans as well, and you give them AI so advanced, that they become self aware and they are also able to learn and grow.
You are God to them. Since you were around before creating their universe, you were always there. This entire world is all within a little hard drive on your computer. It's constantly running and you check back from time to time to see how it's going and occassionaly intervene with things.
Imagine if our situation is the same, we are merely a computer game/program in some computer and not even apart of the real world.