What does 2 Chronicles 20:15 mean?

He said, 'Listen, everyone in Judah and Jerusalem, and you too, King Jehoshaphat. This is what God says: Don’t be afraid or discouraged because of this huge army. The battle isn’t yours—it’s God’s.' 2 Chronicles 20:15 - Modern Text Bible

(God reassures them that they don’t need to be afraid, because he’s taking responsibility for the outcome.)