Widget areas (or Sidebars) usually contain widgets that an administrator of the site can customse. By default Rebecca offers the following widget areas:

The main sidebar can be enabled or disabled on pages and posts, and displayed on the left or right side of the content.  Beside the main sidebar there are other widgets zones as you can see in the image above. In the following image you can see where the other widget areas are positioned:

In the theme settings, under the sidebars options, you can create custom widget zones that can be used on individual pages or posts instead of main sidebar.

Or instead of ‘Below page content sidebar’: