What does it mean if I'm designing an environment and I want to test it in the game engine, to see what its like to "run" [or fly] around my environment? I heard an instructor say that exact thing in a Maya training video and I'd like to know more about "How Game Engines and Maya are related to each other." He stated this would be done to see how things look in "size" (e.g. I assume he meant: 'How big is the cathedral, bridge, wall, building, etc.').

I've tried to research such information but it's too complicated, and detailed. I just want a simplistic response to my query. Thanks to everyone willing to help and not criticize my question.