Stabsfeldwebel was an NCO rank of the German Third Reich Wehrmacht, present in the Heer(army), Luftwaffe(airforce) and Kriegsmarine(navy), though not having the same meaning in all three.
A Stabsfeldwebel of the Heer or Luftwaffe is equivalent approximately to a sergeant major in the current day US Army, or a SS-Sturmscharführer in the Third Reich Waffen-SS. In the Kriegsmarine, however, Stabsfeldwebel signified a higher rank, equivalent to an Oberveldwebel of the Heer or Luftwaffe, an SS-Hauptscharführer of the Waffen-SS or a master sergeant of the current day US Army.
For a more complete overview of which ranks existed, see Wehrmacht ranks.
The page that helped me with this writeup is no longer on the internet, but it used to live at www.wehrmacht-association.com/Open_Pages/ranks.htm