In Drupal, it shows the hierarchy of the page currently viewed.

Breadcrumbs is a navigational aid which allows a user to realize the position of the current page in the site hierarchy.

There are several ways to customize the breadcrumbs theming, 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.