What does Judges 5:20 mean?

According to this part of a celebratory song (Judges 5:1), the kings of Canaan entered battle against vastly overmatched Israelites (Judges 4:12–13). The Canaanites approached on seemingly unbeatable iron chariots (Judges 4:1–3). It's possible they held vastly superior numbers. The Israelites were under-armed (Judges 5:8) and under-trained, at best. It's not surprising the Canaanites expected to collect great spoils from the Israelites.

As explained here, however, the Lord turned the tide of the battle before it even began. Deborah poetically describes the stars fighting from heaven against Sisera and his army. This reference suggests two things. First, God was considered to be the only One in control of the heavens; if "the stars fight" for Israel, it means the Lord is taking action on behalf of His people. She is not actually saying the stars did something in and of themselves, but instead uses poetry to communicate that all of creation was aligned to favor the Israelites.

Second, references such as this also imply something about nature. As the next verse shows, this might have been a major part of Barak's victory over Sisera's forces (Judges 4:14–16). Scholars suggest the heavens broke loose in a furious downpour of rain over the valley, flooding the usually tiny Kishon River. This would turn it into a torrent—a flash flood—sweeping away many men. It would also make the field a muddy mess, incapacitating heavy iron chariots.