On the Mac you can have two different screens when you are connected to a projector or video monitor. Use the System Preferences Display pane to control how you want to use this second screen. You can either position the second screen to the side of your main screen or mirror the first screen on the second.

The picture below shows the System Preferences and you can note the Displays pane which is on the left side:

From this screen you would click on the Displays pane to get the “Arrangement” tab. The picture below shows you the choice where you have two separate screens and the secondary screen is above the primary.

If you are using a projector and wish to have the same images displayed in both places, check the “Mirror Displays” box as shown: