Using 'Export List (backup)' option you can download a file (.xml) containing a backup of your list (structure and data) that you can store and keep in case you need to recover, make a duplicate of your list or migrate among different Joomla sites.
Just use the icon and save as .xml file. In order to recover a backup use 'import list' button on the top toolbar, you will be prompted to select the xml file to recover, structure and data will be loaded inside your list manager.
If you just want to duplicate a list structure, select the list with the checkbox next to its name and use 'Copy' button of the toolbar. This action will add a new list to your set, same fields that original list, and adding (*) to its name to locate it easily as a copy, you can change its name afterwards from the general configuration of your new list.