Sidebar: This sidebar is the default sidebar area. This sidebar area is display at blog page, archive pages, single blog pages, 404 page, search page and other pages.
Primary Sidebar: This sidebar is displayed below the banner of frontpage section. You can add widgets that you want to display beside the secondary widget.
Secondary Sidebar: This sidebar is also displayed below the banner of frontpage section. You can add widgets that you want to display beside the Primary widget.

If you want to add a widget area in the footer then you must add widgets in Footer Widget 1, Footer Widget 2, Footer Widget 3, Footer Widget 4.