This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
This TYPO3 extension adds a Bootstrap Collapsible Panel to your tt_content Elements in the "Page" module view. It can be expanded and collapsed and holds visually relevant properties of this element like the "spaceBefore", "spaceAfter", "layout", ...
If you rely a lot on these properties regarding your layout it is often very cumbersome to open up each element, memorize the settings and then open another element and compare the values.
Now you can expand all panels on each tt_content element to clearly see the various settings and differences. This makes it easier to achieve a consistent frontend layout.