This tab provides options for your comming soon page when your site on maintainance mode.
Look at sceenshot below, you can see that there are several useful options such as content customization, display countdown timer and subcribe form, social list link and enter subcribe form shortcode.
For subcribe form shortcodes, please install and activate Mailchimp for Wp plugins and then navigate to Mailchimp For WP > Forms > Get Shortcode. After that paste shortcode in this option.