What does Psalm 18:42 mean?

Hyperbole is the literary term which means "exaggerating for effect." This was an extremely common practice in ancient literature. A modern equivalent would be someone saying, "I've told you a million times," or "this suitcase weighs a ton!" The speaker doesn't mean it literally, and the one hearing them understands the metaphor.

Still praising God's intervention, David writes that he beat his enemies "fine as dust before the wind." Dust is no match for the wind. The wind blows dust swiftly and far according to its velocity.

David's disdain for his enemies was equivalent to what he felt about trash or useless waste. Just as people swept garbage or grime into the gutter, David flung his enemies aside like waste. Dust and dirt thrown into the street are trampled and completely ignored by those who walk on them (Matthew 5:13). Although God loves everyone and gave His Son to die for the sins of the whole world (1 John 2:2), someday He will cast aside for eternity all who reject His offer of salvation (John 3:36). Revelation 20:11–15 pictures the great white throne judgment: a moment at the beginning of eternity which reveals that all whose names are not found written in the book of life will be cast into the lake of fire.