Symbolism, Imagery, Allegory
When the Aramean army tries to hunt down and kill Elisha in 2 Kings 6, he and his servant are stuck on a mountain with the Arameans closing in. The servant (who may be the same servant as Gehazi, mentioned in the other tales about Elisha) is worried that they won't be able to get out of this.
Elisha asks God to open the servant's eyes and suddenly the servant is able to see that a divine army is surrounding them on the mountain, far more powerful than the Aramean forces: "So the Lord opened the eyes of the servant, and he saw; the mountain was full of horses and chariots of fire all around Elisha" (2 Kings 6:17). Thanks to the power of God, Elisha is able to make the whole Aramean army go temporarily blind, before capturing them, restoring their sight, treating them to a feast, and releasing them.
Basically, the divine army symbolizes God's power. No matter how many physical weapons are arrayed against the people who follow God, a single act of divine power can shut it all down. God has infinite firepower on his side. Though in this case, interestingly enough, God doesn't use that firepower to slaughter Israel's enemies. He uses it to non-lethally disarm them, and then release them—with a great feast beforehand.