The color chosen will be the background color for your tabs and accordion body - the main content area.
This setting in combination with the 'header background color' gives 2 color background schemes to your templates.
In the case of tabs, the 'body background color' also affects the headers: the non-active tabs will get this color as a background as well (while the active tab will get the 'header background color').
Have a look at the color schemes demo for examples of how these colour combinations come together.
Body background color setting is a basic configuration option.
Here is the list of all basic configuration options.