"The multitude will be defeated, and they will turn their backs [in flight]," meaning: on the day of Badr.
"The matter descends," meaning: the revelation. "Between them," "between the heaven and the earth." "That you may know" through this revelation "that Allah is over all things competent and that Allah has encompassed all things in knowledge," for nothing falls outside of His knowledge.
Muhammad said: "[In knowledge]" is in the accusative case as an emphatic verbal noun (masdar). The meaning is: He has known everything with knowledge. This part has ended with the praise of Allah.
﴿سَيهْزمُ الْجمع وَيُوَلُّونَ الدبر﴾ يَعْنِي: يَوْم بدر