If you're connected to two or more displays, you can mirror your main display on the second display. Owning a MacBook, you will see your F7 key being embedded with icon showing two stacking blocks, that's the shortcut to activate mirror display.
Apart of that, you can also slowly set your Mac to mirror display via Display Preference Panes:
If you've more than two displays, enabling this feature will make all your displays showing the same image as your main display. Let's say it's not what you want; You want to mirror your main display only on your 2nd display and left the 3rd display intact. It's quite simple:
Some articles taken from our resource base, tightly related to current article, to empower you with more knowledge on tweaking the most out of your Mac.