Most of the information on your Mac is displayed in windows, including Finder windows and app windows.
When you have multiple windows open, only one is active. When an app window is active, the menu bar contains the app’s name. Some windows that you open within apps, such as the Fonts window, always appear in front of other windows.
Move, resize, and minimize windows
Move a window: Click the window’s title bar, then drag it where you want it.
Manually resize a window: Drag the window’s edges (top, bottom, and sides).
Maximize a window: Hold down the Option key while you click the green maximize button 
in the top-left corner of an app window. To return to the previous window size, Option-click the button again.
You can also double-click an app’s title bar to maximize the window (as long as the option to do so is set to “zoom” in Dock preferences).
Minimize a window: Click the yellow minimize button 
in the top-left corner of the window, or press Command-M.
You can set an option in Dock preferences to have a window minimize when you double-click its title bar.
Some windows can’t be moved or resized, and may require that you perform an action or answer a question before you can continue with a task.
Quickly switch between apps
If multiple apps are open, it may be difficult to find the one you want. Here are shortcuts you can use to move among apps.
Quickly switch to the previous app: Press Command-Tab.
Scroll through all open apps: Press Command-Tab, continue to hold the Command key, then press the Tab key repeatedly. When you get to the app you want, stop and release the keys.
You can also press Command-Tab, continue to hold down the Command key and use the mouse pointer or arrow keys to scroll.
Resume work without switching apps: Press Esc (Escape) or the period key.
Here are other tasks you can do after pressing Command-Tab and holding down the Command key:
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Hide a selected app: Press H.
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Quit a selected app: Press Q.
Close windows
Click the red close button 
in the top-left corner of the window, or press Command-W.
When you click the close button in many apps, such as Photos or Notes, you quit the app. Other apps, such as Safari or Mail, remain open when you click the close button (only the window closes). To quit these apps, click the app’s name in the menu bar, then choose Quit [App]. For more information, see Quit apps.
With many apps, such as Calendar and Mail, you can work with the app in full screen—the app expands to fill the entire screen—or you can open a second app and use both apps side by side in Split View. For more information, see Focus on apps in full screen or Split View.