This means that if you’re viewing a web page in Safari, for example, and you double-click the title bar, the window will expand to the top and bottom of your screen, but only as far left and right as needed to fit the website’s contents. In most modern apps, the window will expand as large as it needs to (up to the maximum area of your screen) to fit the window’s contents. In general, however, “zoom” will make the window bigger. The “Zoom” option is a bit confusing, because it doesn’t work the same way for all applications. So, double-clicking in an empty area of this space will produce one of the following two results: zoom or minimize. First, however, what does Apple mean by “ title bar,” anyway? That’s referring to the empty gray area at the top of the windows in most programs, where apps will house their various buttons and such.
The first option lets you decide what will happen to a window when you double-click its title bar. Change What Happens When You Double-Click the Title Bar
