The Son of David
Jesus wanted the crowd to think about something that the teachers taught. They said that the Christ was the son of David. (See my notes about Mark 8:27-30 and Mark 11:1-11.)
Jesus did not say that the teachers were wrong about that. Perhaps the teachers meant that the Christ belonged to David‘s family. That was right (Isaiah 9:7; Isaiah 11:1; Matthew 1:1).
But the word "son" shows the idea that the "Father" is more important. And David is not more important than the Christ.
David knew this. God‘s Holy Spirit helped David to write about the Christ in Psalm 110:1. There, David calls the Christ "my Lord," which means "my master." Nobody calls his son, "my master."