In Drupal, it shows the hierarchy of the page that the user is currently viewing.
Breadcrumbs is a navigational aid which allows a user to realize the position of the current page in regards with the site hierarchy.
Drupal uses the
active-menu-trail to calculate the breadcrumb which may be presented by the current theme. There is a theme variable setting which controls whether the breadcrumb is displayed.
There are several ways to customize the breadcrumbs theming, including using
drupal_set_breadcrumbs() in a custom theme or module, and there are plenty of contributed modules which affect the breadcrumb. This is very useful for sites that the breadcrumbs cannot be calculated in a straightforward manner from the menu items.