Footer layout options can be found in Customize > Layout > Footer.
The Footer Width option is the outer container of your footer, including your footer widgets and copyright area.
Setting this to Full will make your background color/image span the entire width of your screen.
Setting it to Contained will contain it to the width of your container, set in Customize > Container.
Inner Footer Width
The Inner Footer Width option refers to the inner container which holds the content of your footer (Footer widgets, copyright area).
Setting this to Full will make your footer content span the entire width of your screen.
Setting it to Contained will contain your footer content to the width of your container.