Great question, Ken. Psalm 102:18 (ESV) reads: "Let this be recorded for a generation to come, so that a people yet to be created may praise the LORD."

The "this" that must be written down is the testimony of God's sovereign, redemptive intervention — specifically what the psalmist has been declaring in the surrounding verses (vv. 12–22). The content includes God's rising to have compassion on Zion (v. 13), His appearing in glory (v. 16), His regarding the prayer of the destitute and not despising their plea (v. 17), His looking down from His holy height to hear the groaning of the prisoner and set free those appointed to death (vv. 19–20).

In short, what is so important that it must be written down is the record of God's faithfulness — that He hears, He acts, He delivers, and He restores. The psalmist understands that this is not merely a private comfort for the present sufferer but a testimony with eschatological reach: "a people yet to be created" will need this witness so that they too may praise the LORD.

There's a beautiful redemptive-historical thread here. The phrase "a people yet to be created" (עַם נִבְרָא) uses the verb בָּרָא — the same verb of divine creation from Genesis 1:1. The psalmist isn't just talking about biological descendants; he's pointing toward a people whom God Himself will bring into being. For the Christian reader, that finds its fullest resonance in the new-creation people gathered in Christ — those who were once "not a people" but are now "God's people" (1 Peter 2:10), created in Christ Jesus (Ephesians 2:10), born again to a living hope precisely so they might declare the praises of the One who called them out of darkness (1 Peter 2:9).

The psalm insists that God's acts of deliverance are too important to die with the generation that experienced them. They must be inscripturated — written down — so that the praise of the LORD never falls silent across the generations.