The explanation below is excerpted from the link above. If you want to learn more about it, please visit the link.
This box will likely be your most-used box. You can add any kind of content you want within this box - not just Rich Text/HTML including:
Content you've added:
Books from the Catalog
Google Search Boxes
Widgets from Springshare Products
You can add various types of content all in one box. You reorder content within the box as well! Move a whole group of links from one place to another, or simply move one link from one group of links to another within the same box.