This was an Oberkommando der Wehrmacht (OKW, "High Command of the Armed Forces) order, transcribed in full here. It built upon orders given at the start of Barbarossa, including the June 6th Commissar Order, which generally gave German soldiers broad leeway (indeed, the obligation) to kill prisoners, political officers, and civilians. These orders were reflective of the Nazi conception of the war as an absolute war, a Vernichtungskrieg or war of annihilation. In addition to the these orders from OKW, individual army and division commanders issued their own orders permitting and encouraging brutality. For instance, the 12th Infantry Division issued orders around the same time as the September 8th OKW order that "Prisoners behind the front-line.... Shoot as a general principle! Every soldier shoots any Russian found behind the front-line who has not been taken prisoner in battle." (Bartov 84) By the time this order was issued, however, mass shootings of prisoners of war had already happened in some sectors. The commander of XLVII Panzer Corps, Joachim Lemelsen, noted on June 25th (three days into the campaign) that the men under his command were shooting POWs of their own accord. He felt compelled to issue an order instructing that civilians and men in uniform who had surrendered already should not be shot wantonly; however, accused partisans and political commissars were still fair game under this order. (Bartov 85) Similarly, on July 5th, 16th Army's headquarters warned that POWs in work-gangs should not be shot. (Bartov 85) From these examples we can see that this order was intended to restore the semblance of control to an army that was already practicing wanton violence; however, this semblance of control was not intended to stop violence but rather to put it back within the bounds of the command structure.

Chapter 3 ("The Perversion of Discipline") of Omer Bartov's Hitler's Army is good further reading on this.