To quit (close) a Mac app normally, choose Quit from the app's menu in the menu bar, or press Command (⌘)-Q. If the app doesn't quit, follow these steps to force the app to quit.
How to force an app to quit
Press these three keys together: Option, Command, and Esc (Escape). Or choose Force Quit from the Apple menu in the upper-left corner of your screen. (This is similar to pressing Control-Alt-Delete on a PC.)
Then select the app in the Force Quit window and click Force Quit.
You can even force the Finder to quit, if it stops responding: Select Finder in the Force Quit window, then click Relaunch.