It can be interpreted in at least two ways.
The fighting of the stars
can be regarded as a poetic embellishment of the thought that it was the Lord Himself who fought against Sisera.
Some natural phenomenon frightened the enemy.
It could very well be the latter. This verse seems to suggest that the Lord helped Israel by means of forces in the atmosphere. It could have been a very severe thunderstorm, possibly accompanied by hailstones (see Joshua 10:11, that threw the enemy into confusion or panic and also caused the Kishon to overflow.1
20 From heaven the stars fought, from their courses they fought against Sisera.