The category description box gives the shop owner the opportunity to describe and promote the category on the category's page above the items. This box is a standard Joomla! content box, and can be used in exactly the same way a Joomla content box would be used.
NOTE: Initially, the description box cuts off the right hand side of JCE editor by default to save space when not used. This is normal, and not a bug that you need to raise a help desk ticket about. Click on the lower right corner as indicated in the image to enlarge content area to full size while editing.