This window is a once popular way to show the focus of the content on the website. The most used tags are larger than the less ones. You can click on a tag to get to the search results page narrowed down by a chosen tag.
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.