Extra Content (EC) is a term to denote areas of your page which aren’t provided in RW by default.
A standard RW page - at its most basic - allows you to place content (text, images etc) in the page’s centre, or main content area; in sidebars; perhaps in a footer; and have - usually - images in/as the banner. The default page also allows you to control the menu/navigation buttons, tabs, lists etc.
Imagine a tiny area at the top right of the entire page, or immediately above your vertical navigation between the menu and the header image, or stretching all the way across the page butted up against the header/banner. Into any of these areas you might want to put such things as today’s date, a secondary logo, a link to your own site-building company on a client’s own site.
These are Extra Content areas; they are controlled separately and specifically by themes which support EC. Many modern themes provide for, say, 6 EC areas to be optionally ‘activated’ and used (=filled with content of your own).
The way in which (or even whether) Extra Content is implemented always differs from theme to theme. So do the ways in which you access EC… Stacks4Stacks EC stack is one way to make it easy.
Many EC-supporting Themes will place a distinct (bright red, yellow, e.g.) border around an ‘activated’ EC area to show you where to is - as well as describing its location on the page. This border does not display on publication.