The Status Bar is the section at the bottom of the editor displaying information about the HTML elements of the document. It is an advanced feature aimed at users who want to have more control over the source code of their document.
The status bar always shows a hierarchy of HTML elements for the element that currently has the focus.
This is what the status bar looks like:
The mouse cursor is now located inside the word "Second", so the status bar shows the following hierarchy of HTML elements: body (main document element), ol (ordered list), li (list element) and strong (bold font).
However, most of the same information can also be deduced based on the main CKEditor toolbar since the formatting options that were applied on the active element are highlighted.