You can add various pieces of metadata to a theme. Some are stored in the about.json file, and some are stored in the locale files.
about.json 
file format info
name (string, required): The default name for the theme when installed. This can be changed by admins after the theme is installed
component (boolean, default false): whether the theme should be treated as a component
licence_url (string, optional): a URL for a license file. A link to this will be displayed in the admin panel. Most themes use this to link to their license file on GitHub
about_url (string, optional): a URL which contains more information about the theme. A link to this will be displayed in the admin panel. Most themes use this to link to their Meta topic
authors (string, optional): A string to describe the author of the theme. Displayed in the admin panel.
theme_version (string, optional): An arbitrary string to describe the version of the theme. Displayed in the admin panel
minimum_discourse_version (string, optional): the earliest discourse version which this theme is compatible with. If it does not match, the theme will be auto-disabled. Should be in the format 2.4.0.beta1
maximum_discourse_version (string, optional): the latest discourse version which this theme is compatible with. If it does not match, the theme will be auto-disabled. Should be in the format 2.4.0.beta1
locale files (e.g. en.yml) file format info
theme_metadata.description: A localised description of the theme. Displayed in the admin panel
theme_metadata.settings.setting_name: A localised description of setting_name, displayed below the theme setting in the admin panel