What does Psalm 18:12 mean?

According to David's poetic description of how the Lord rescued him, brightness went before the Lord and hailstones and coals of fire broke through the clouds. The biblical account of David's battles with Saul, or his other enemies, don't refer to literal earthquakes or fire (Psalm 18:7–8), nor hail, nor pulling David out of the water (Psalm 18:16). This is part of David's continuing imagery about the power God expressed in rescuing David. Coals of fire breaking through the clouds might be a depiction of bolts of lightning or even a meteorite shower.

Hail is rare in that area of the world, but the Bible mentions hail occurring in connection with God's judgment. The Lord sent heavy hail onto the land of Egypt as the seventh plague (Exodus 9:18–19). He also sent hailstones on the five kings of the Amorites and their forces who came against Joshua and Gibeon (Joshua 10:11). Haggai 2:17 attributes a judgment of hail to the Lord to persuade His backslidden people to return to Him, but they did not turn to Him. Revelation 16:21 reports that under the seventh bowl judgment in the tribulation hailstones weighing about one hundred pounds each fall from heaven upon the wicked.