The email "body" is the main background of your email message. This background is behind all the content you add to your template.
It's like a special table which contains every other table you add to your template later. You can change the settings of your email body under the "Defaults" tab in Chamaileon.io.
The email body can have:
Background color - You can give a single background color to your template.
Single image - You can either add a large single image (generally not advised since your email weight can get really heavy)
Pattern - Or use some lightweight pattern instead which repeats in the background of your email.
Note: You need to use "repeat both" property for your background images / patterns to make them work in Outlook 2007, 2010, and 2013.