They're talking about a window sized to the maximum space as if you click and dragged its edges out as far as they can go or just used a tool like Divvy.

You're talking about the "fullscreen" feature which I always found very weird. For example, ever forget your video was "fullscreen"ed as you try to alt-tab to it only to realize it's a 4-finger swipe to pull it back up. Making the user have to differentiate will always be bizarre to me.