Genesis supports 3 different layouts including both sidebars on either the left or right of the main content. In this configuration it can be useful to have a sidebar-top region that spans the width of both sidebars, but sits on top of them.
An example of when you might want to do this would be to display a fixed height banner or contextual ads. Its not good for content that will change in height as it could then overflow the divâ€”while the fixed height limitation could be overcome, this tutorial is meant for beginners and is a simple strait forward explanation of how to add new regions in Drupal and how to position the new sidebar-top region using CSS.
This tutorial shows you how to do this in the Genesis starter theme for Drupal 6.