If you have an external screen you can use this as a second screen in many different ways.

When choosing from the Multiple Display drop down you can have your displays behave a few different ways:

Duplicate desktop
 The image displayed on your main screen will be the same as displayed on the other screens that are connected to your computer.

Extend desktop
 The external screens will behave as additional screen real estate and can be used for working on in addition to your main screen.

Disconnect this display.
 This will switch off the display that is selected. (Displays are selected at the top and appear blue when clicked.)