What does Psalm 22:31 mean?

In this verse David foretells that a generation will proclaim the Lord's righteousness to a people yet unborn. The preceding generation will declare that the Lord has done it; that is, provided salvation. A thousand years after the life of David, the Messiah Jesus died on the cross to provide salvation by shedding His blood for us.

Jesus uttered the first words of this psalm from the cross (Psalm 22:1; Matthew 27:46). Christ's final words from the cross were similar to the last words of this same passage: "he has done it." At the moment of death, Jesus cried out, "It is finished" (John 19:30). The Greek word for "it is finished" is tetelestai, which implies something has been completed, paid, fulfilled, or reconciled. In the first century when a painter put the final brush stroke on a canvas and was satisfied with what he saw, he might say "Tetelestai!" His painting was complete. Nothing needed to be added to it. Tetelestai was written on a loan when the last payment was made.

Jesus' declaration of "Tetelestai!" means "nothing needs to be added to what I have done. Paid in full!" Salvation, then, is not a matter of what anyone can do to earn salvation, but a matter of what Jesus has already done.