A Child Theme is used if you want to make some changes to CSS styles or functionality of the theme and you want to keep receiving updates on the main theme.
You can install a Child theme the same way you have installed you main Crane theme – via FTP or via WordPress admin panel. It comes with blank files, so you should add your custom code directly to Child Theme files to make changes appear on your site. These changes won't be overwritten parent theme during Crane updates.
Advanced Users: If you are going to make changes to the theme source code please use a Child Theme rather than modifying the main theme HTML/CSS/PHP code. This allows the parent theme to receive updates without overwriting your source code changes. See crane-theme.zip in the main folder for a sample.