Breadcrumbs are a way to improve the user's awareness where in a (big) file he is currently editing. It is essentially a "horizontal tree" shown at the top of an editor that reflects the hierarchical structure of an editor's contents. Niko has made an MPS extension that displays a breadcrumb in an MPS editor. By clicking on a label the user can jump to the corresponding level of the containment hierarchy. By clicking on the little triangles one can see the remaining nodes in the hierarchy.
To use breadcrumbs in an editor, only two simple steps are required. Nodes that form the hierarchy (and should show up in the breadcrumb) must implement the interface IBreadcrumb. The root of the editor that should show the breadcrumbs must include a breadcrumb editor cell at the top.
The breadcrumbs are available as part of the mbeddr.platform.