Breadcrumbs are the small > cascading > links that you can see at the top of every page. They allow visitors to see exactly where they are on your website and back-track their way up the page and category hierarchy. This functionality can be turned off via the admin panel.
If the page you are visiting has other sibling pages under the same parent, a quick click on the final breadcrumb link will show a list of those siblings as links - it essentially acts as another mini navigation menu.
Should you wish to prevent visitors clicking on empty parent breadcrumb links, but still want to show their child pages, you can configure the breadcrumb link for that page to always be un-clickable.