Let's say you closed the browser with 5 tabs open. When you restart it, only 1 of them will be in the foreground (active), and that's the only one that's loaded. The background tabs (i.e. the other 4) are only loaded when clicked. Some people prefer this to speed up browser launch time and to save memory. I'm not actually sure which way it's does now that there isn't a choice (they must have picked one way or the other).