Semmelknödel is a German word which means bread dumpling. Semmelknödel is made mostly with cubed white bread/roll called Semmel.
Semmelknödel is usually served as a side dish (Beilage in German).
Semmel is a word used in Austria and Bavaria for white bread or roll which is used not only for making Semmelknödel, but also to make Semmelbrösel (breadcrumbs) or Semmelmehl (white breadcrumbs/bread flour)