The Monitor Output allows you to send the entire PVP workspace to an external screen.  This is helpful in that you can see a larger preview of the full PVP workspace. 

To turn this option on, you will click on the Master Layer Strip and then will see a dropdown for Monitor in the Sidebar under the Output Status option. 

Here, you can choose which output you wish to use.  This can be a system output, sent through a supported SDI device or be sent over NDI or Syphon.  This feature is limited to sending to only one output and will need to have the Output Status toggled ON for the monitor to appear.