Structure

Under-linked

Pages nothing links to, pages only the menu links to, and pages below the site's average.

Pages the site does not support

Under-linked lists the pages your own links do not support enough, in three lists you switch between in the rail. All three leave out the start page and count only pages that answered with an HTML page.

No inbound links
Nothing on the site links to the page, not even the navigation. The crawl found it through the sitemap, a redirect or a canonical tag, so a visitor clicking through the site never reaches it. Largest pages first.
Navigation only
The page is linked, but only from navigation, header or footer blocks that repeat across the site. No page recommends it in its content. Sorted by how many navigation links it has.
Below site average
The page has at least one link from another page's content, but fewer than the average page of this site. The bar shows its count against the average; the pages furthest below come first.

The site average is the mean number of in-content inbound links over all crawled pages except the start page. The view states it at the top, so you know where the line sits for this site.

The Under-linked view: the site average, then pages with no inbound links
The site average at the top, then pages nothing links to, found through the sitemap, largest first.

Fixing them

Open a row and it starts on its Suggested tab: the places on other pages where a link to it would fit. The same pages lead the Suggestions view, pages without inbound links first.

A page with no inbound links is not always a mistake: a landing page for a campaign, for example, may be meant to be reached only from outside. The list shows every such page; you decide which of them should be part of the site.