Resource Overview Reports allow you to compare different resources within a resource group. For example, you can compare the status or trends of all ArcGIS servers using a multitude of metrics.
If you click a resource group in the navigation section, the navigation section updates to show the sub-list of items in your selection.
With a group-specific resource list shown in the navigation section, Reports shows an overview of those items in either the Status tab or the Trends tab. This is a Resource Overview Report.