Each folder that is included in the navigation needs a page named 'index' at the top of the folder sort order. This is the 'home' page for that folder. When someone uses the breadcrumb links at the top of the content or the left side navigation they will go to the index page of that folder when it is clicked on.
Use the index page to convey, explain, or summarize the folder’s concept and/or list an explanation of the sub pages.