Dim screen mode allows a Presence camera to consume less electricity and be more stealth. Dim mode is invoked by tapping the “Dim” button on your camera screen. Don't worry, all Presence functionality works while you're in the Dim Mode.
The closest that the device can become totally Dim is if brightness is adjusted to the minimum in device settings. If the camera is being watched the Free version still displays a message when it is viewed. The Presence Pro version can hide this message.