The SITE DETAILS section contains the Site name, Slogan, and Email address fields. Depending on how an installation is completed, there will always be values for both Site name and Email address already filled in:

The Site name field is used by Drupal to display the website's name.
The Slogan field is used for displaying a website's tagline.
Both the Site name and Slogan can be toggled on and off from within the Site branding block used by our theme, and generally displayed within the Header region.
The Email address is used as the from address for all automated emails sent by Drupal. These emails can be for registration, retrieving passwords, and other notifications. This Email address is also used to notify us when an update is available for the core or contributed modules.