What is the meaning of "Gerafam"?
Rashi: He swept them out of the world, just like a broom is Goref (sweeps) ashes out of an oven.
Radak (20): The war was near Nachal Kishon. They entered to flee or to fight Yisrael (or to cool off - refer to 5:20:2:3), and the chariots broke, like in Egypt "va'Yasar Es Ofan Markevosav" (Shemos 14:25), and the soldiers fell in and drowned.
Why is it called Nachal Kedumim?
Rashi citing Pesachim 118b: It is the river that was a guarantor mi'Kedem (from of old. When Hashem told the sea to spit the Mitzriyim onto the dry land, it did not want to. Hashem made Nachal Kishon a guarantor, that later He would return even more (900 chariots, in place of Pharaoh's 600).
Radak (20): It is a river that was there from before, as if it was created in order to avenge Hashem's enemies - "Kol Pa'al Hashem l'Ma'anehu" (Mishlei 16:4).
Malbim: It already swept these camps previously. The Shirah equates Mei Marom (refer to 5:19:1:2) with Nachal Kishon.
What is the meaning of "Tidrechi Nafshi"?
Rashi: My feet will trample on the strong ones of Kena'an.
Radak (20): Whereas Sisera's soldiers fell in the river and drowned, Yisrael walked through without mishap, for from Shamayim, they fight for me.
Lev Yitzchak: Targum Yonasan is Taman Dushishas Nafshi Ketilei Gibareihon bi'Skof.. See Meturgeman, Dash (who says that also Dushishas refers to threshing).
Malbim: My soul tramples the water, which is Oz (fierce), and brings me through. At the time that it swept Sisera's camp, Barak and his camp passed through.
