These options allow you to set the page background color(s). Page Background is pretty self-explanatory.
Regarding linear gradients, a few points to keep in mind:
Those familiar with the previous generation CE2 plugins may be distraught to find that we have eliminated options for background images. In developing CE3, we have determined that the inclusion of background images creates a lot of unnecessary overhead for the plugins, and so we have streamlined these options away. If you want to include a background image, the best way to do it is to use custom stylesheets via PHPlugins. Please see the PHPlugins section of the wiki for more information.