Well, stating it in simple words ‘One Click demo import’ is a way by which you can import all the theme demo data in a single click. This makes it possible to import all the WordPress demo content including Customizer data, settings, used plugins data etc. in a matter of couple of minutes.

Demo import option lets you import the following:
1. It imports all posts, pages, custom post type content, images
2. Imports menus, custom fields, Categories, tags, and comments
3. Imports all the data of the plugins used, widgets, Customizer settings & the front page.

By importing the theme demo content, users get a quick way to customize the theme and build a stunning website using a WordPress theme. Users get complete demo website data with all the Customizer options, plugins and widgets already working. Now, you can quickly customize the site with your own content.

Only  Theme-Options will not be imported automatically. You will be able to import the Theme-Options separately by following the steps written here.