The Admin Stats tool, along with the Patroller Stats and Steward Stats tools, are for listing users in a given project with specific rights, and listing the number of actions they took using those rights.
It can be a way to see which members of a community have been the most productive. However, it should be noted some actions are hidden using suppression and log deletion due to sensitive contents, but these actions will not be counted in XTools since they have been removed from public view.
The "Admin" group are any users capable of making admin actions. These actions are listed on the initial form, and you can selectively chose which ones you want to query for. Similarly for the "Stewards" group, there are a different set of actions to choose from relevant to Stewards. Finally, the Patroller Stats has only rights relevant to Patrollers.
The Summary section lists the requested information along with the number of current admins and non-admins with admin actions. The latter could happen for example if a steward were to delete a page on a wiki where they do not have admin rights.
This section lists all users who have made the requested actions, sorted by the number of overall actions they made.
The "User groups" icons are the community standards as pulled from c:Category:Wikimedia user rights icons. You can hover over the images to see which user group they refer to. Icons were necessary for optimal readability.