The people living in the house are the "active" objects—they're
the ones using the various rooms, watching TV, cooking, taking
showers, and so on.
We'll soon see that's how threads behave.
Single threaded
If you've ever lived on your own, then you know what this is like—you know that you can do anything you want in the house at any time, because there's nobody else in the house. If you want to turn on the stereo, use the washroom, have dinner—whatever—you
just go ahead and do it.
Multi threaded
Things change dramatically when you add another person into the house. Let's say you get married, so now you have a spouse
living there too. You can't just march into the washroom at any given point; you need to check first to make sure your spouse
isn't in there!