The "Song of the Nibelungs" is a German epic poem from the early Middle Ages, presumably around 1200.  Like many epics of the time, including Beowulf, this poem depicted the theme of duty and chivalry.  Siegfried represents the chivalric code.  He courts Kriemhild properly and marries her.  The prisoners of war are treated less like prisoners and more like guests, as was the code of the time.  Kriemhild has a duty to pay to Siegfried, believing she must avenge his death because he was her husband and it is her job.  Her brothers have a duty to their sister and are bound by this duty to help her.  Rudiger is put into conflict because of his conflicting duties to Etzel and the Burgundians. 

The social significance is closely tied to these plot and character elements.  Feudal traditions and laws valued strength and bravery.  Although the chivalric code emphasized many points of moral behavior, a heroic person with questionable ethics was still highly valued.  Consider Hagen in the story, who is respected for this strength even though he is a murderer.