This window is used to place html-formatted text into any zone. You don't need to know HTML to put the text in. However, placing images or complex elements will need such knowledge. You can also use this window to place HTML-code of counter, soclal-media widgets etc. on your page.
Example of HTML-formatted text is shown below.
If you are an experienced in HTML, CSS and have some programming skills, you may edit the templates for you site. Just add /_module/website/themes/editor to you site address to get to the Cornerstone CSS-editor.