You can use either mirror mode (to show the same content on all displays) or extended mode (to use them as one large, continuous desktop) on a MacBook. To change the setting, go to the Apple menu > System Settings > Displays, click your external display, and select either "Mirror for Built-in Display" or "Extended display" in the "Use as" menu.  

This video demonstrates how to switch between mirroring and extending displays on a Mac: