The most common reason for this happening is because your Joomla Administration session has expired.
Solution: refresh the page and log back into your administration area and try the task again.
Think of Configurator as "template parameters" on steriods. It is accessed via your Joomla admin & provides you with an intuitive interface to configure the structure of your site, customize your themelet & choose which features you want enabled.
The template is where all the core functionality resides & should never be modified. Thanks to the separation from the themelets, you can easily upgrade the framework without losing any of your changes.
Borrowing from the parent/child-theme principle in Wordpress, themelets are the visual layer that sit on top of the framework. They are mostly made up of css & images, but also support html/layout overrides & custom css/javascript/php.
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