Psalm 33:17, "A horse is a vain thing for safety, neither does he deliver any by his great power," speaks to the limitations of relying on military strength, specifically horses, for protection and deliverance. Here's a breakdown:

"A horse is a vain thing for safety": In ancient times, horses were a primary instrument of war. A powerful cavalry was a significant military advantage. However, this verse declares that putting your trust in horses (and by extension, military might) for security is ultimately futile. The word "vain" implies emptiness, uselessness, and lack of true value.
"neither does he deliver any by his great power": Even the sheer power of a warhorse cannot guarantee deliverance from danger or enemies. The verse emphasizes that true salvation doesn't come from physical strength or military prowess.

In essence, the verse is a statement against relying on worldly power and a call to trust in God instead. It highlights the idea that human strength and resources are limited and cannot ultimately provide lasting safety or salvation.

Here's how we can understand the verse in broader terms:

Trust in God vs. Trust in Self/Worldly Things: The verse promotes the idea of placing trust in God's power and provision rather than in human-made things like military strength, wealth, or political alliances.
Humility: It encourages humility by acknowledging the limitations of human abilities.
Spiritual Warfare: It suggests that true battles are often spiritual, and physical strength alone is insufficient.

In modern terms: This verse can be applied to any reliance on worldly means for security. This could include trusting in:

Military might over diplomacy
Wealth for happiness
Technology for solving all problems

The core message remains: True safety and deliverance come from trusting in God, not in the temporary and limited resources of the world.