Curtain Mode blocks the display on the remote Mac you're connected to. It's useful if you don't want the remote user to see what you're doing, whether because it's distracting or because you're entering confidential information such as a password.
This is what people in front of the remote Mac will see.
This is what you will see in Screens.
You may enable Curtain Mode from your saved “screen” settings, under More Settings. Screens even allows you to customize the message displayed on the remote Mac. Note that this feature is only available when you are connecting to a Mac.
Note: This feature is available only when connecting to a Mac.