The Site Administrator area allows other Administrator users to manage who is currently a Site Administrator on the system.
This area can be reached by heading into the "Site Administration" button as an Admin on the site and then clicking into the "Users" tab, then scrolling to the "Permissions" area and then clicking on the "Site Administrators" link.
The area is laid out with 2 area boxes, the left side is the list of current Site Administrators on the system, while the right side area shows a list of all of the remaining users on the site that are not currently Site Administrators.
To add a new Site Administrator you can search for the user you wish to add by using the "Search" box found below the right side user box to search and filter users on the system and then select the user and click on the "Add" button to move this user to the left side as a Site Administrator.
The same logic applies to remove a Site Administrator, you will need to click on their name in the left side area and then click on the "Remove" button to remove the Site Administrator permission from the users account.