A "gratin" is a French description of a shallow dish of food sprinkled with cheese or buttered bread crumbs and then broiled until crisp and brown. This savory potato soup is garnished with toasted pumpernickel bread covered with melted Swiss cheese.
Pumpernickel is a traditional bread made with a large proportion of rye flour to wheat flour. The loaves are heavy, coarse and slightly sour. A little bit of molasses give pumpernickel its dark color; some bakers add caraway seed or raisins for flavor and texture.