The admin domain resembles the name of your site, which is automatically generated while creating your site.   This domain will always be available even after you create a custom domain or custom URL for your site. The domain needs to be renewed when/before expired to keep your site seamlessly working.  

The admin domain consists of the name of your site plus the suffix '.mindspun.io'.  So if you name your site example then your admin domain would be example.mindspun.io.

The admin domain can be reached through its URL.  The exact URL depends on whether or not you use a custom URL and if that URL has extra path information.  For instance, this site is (of course) a Mindspun \site with the name mindspun-help with and admin domain of mindspun-help.mindspun.io.  The custom URL is https://www.mindspun.com/help/ since it is integrated into the main mindspun.com site.  This makes the site discoverable using the admin domain through the URL https://mindspun-help.mindspun.io/help/.

NOTE: Both of these URLs refer to the same site but the non-admin domain URL is the canonical one.