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What is this and can I access it in my version of Mac OS?
Yes, it is a quick switch between open applications and you can access it by clicking cmd/apple and the tab button. It just cycles from one to the next. You click it again to go to the next one or you can use the mouse to select the one you want if you have a lot open.
People call it the "Application Switcher." You can also quit applications by selecting it's icon and then pressing the Command-Q keys. The arrow keys can also be used to navigate the applications.
Just like Alt-Tab on a PC