Hey Dan Senner,
This setting allows you to set portals that you trust to access your secured content. So if your agency has both an ArcGIS Online and ArcGIS Portal, you can set the ArcGIS Online to trust users who are logged into the specified ArcGIS Portal. This means users will only need to login once!
Hope that clears it up for you!
Thanks,
Ben
If this answer was helpful please mark it as helpful. If this answer solved your question please mark it as the answer to help others who have the same question.