What does Psalm 24:9 mean?

Again, David calls upon the city of Jerusalem to welcome the King of glory. In Hebrew, this is a word-for-word repetition of the proclamation made in verse 7. This emphasizes the importance of the statement, and fits into the poetic structure of the psalm.

It's likely that at this moment, David was returning the ark to Jerusalem from the home of Obed-edom (2 Samuel 6:12–15). The ark was a visible reminder of God's presence. It was constructed to rest in the Most Holy Place in the tabernacle (Hebrews 9:3–4). David was returning the ark to Jerusalem to rest once again in the tabernacle. The King of glory, therefore, would be entering the city.

However, the scene of the ark entering Jerusalem also previews a future event. Before He was arrested and nailed to a cross, Jesus entered Jerusalem. He arrived in humility, but received a spontaneous welcome. "Most of the crowd spread their cloaks on the road, and others cut branches from the trees and spread them on the road" (Matthew 21:8). Those who went before Him and those who followed Him shouted, "Hosanna to the Son of David! Blessed is he who comes in the name of the Lord! Hosanna in the highest!" (Matthew 21:9). Clearly, they were acknowledging that He is the Messiah. However, before long the crowd clamored for His crucifixion (Matthew 27:23).

Nevertheless, Christ will enter Jerusalem again, at His second coming. Then He will arrive as the conquering King of kings and Lord of lords (Revelation 19:11–16).