Episode 58: The cornerstone

In the days just before his crucifixion, Jesus was publicly debating the Pharisees in the temple courts. He asked them if they had read the verse about the stone which was rejected and then became the cornerstone. Of course they had: it was from Psalm 118, and everyone was in Jerusalem for the Passover, at which that psalm would be sung. Later, the apostles Peter and Paul would refer to Jesus as the cornerstone as well. What does this mean?