What does Mark 15:5 mean?

When the Sanhedrin tries to find false witnesses against Jesus, He makes no defense (Mark 14:55–60). Directly after this interview with Pilate, Jesus refuses to say a word in defense to Herod Antipas (Luke 23:9). Jesus does speak to clarify His identity as the Son of God, the Messiah (Mark 14:61–62), and the King of the Jews (John 18:36). He also engages with Pilate to give the governor an opportunity to acknowledge the truth of Jesus' identity (John 18:37–38).

In Isaiah 53:7, the Suffering Servant is described as a lamb that does not open its mouth. In so far as it applies to His impending death, Jesus never "opens His mouth" to defend Himself against false accusations. He does nothing to escape the path to the cross. He speaks, but never says anything to help His own cause. Pilate is amazed. He knows that the Sanhedrin's case is weak, and Jesus can avoid death simply by defending Himself. But He doesn't.

Between Mark 15:5 and 6, Jesus faces another trial (Luke 23:6–12). Herod Antipas, the tetrarch of Galilee and Perea, is in Jerusalem, possibly for the Passover. At the beginning of Jesus' ministry, Herod killed John the Baptist through the machinations of his wife, Herodias (Mark 6:14–29). Herod has had his concerns about Jesus, thinking He is John the Baptist raised from the dead (Mark 6:14–16), and his followers have been conspiring to destroy Jesus from the beginning (Mark 3:6). The Pharisees warned Jesus that Herod wanted to kill Him; Jesus responded by calling him a fox (Luke 13:31–32).

When the two finally meet, Herod asks the questions, the chief priests and scribes spew venom, and Jesus stands silent. Herod had probably hoped for a meaningful conversation, as he'd shared with John the Baptist (Mark 6:20), or at least a miracle or two. When Jesus doesn't oblige, Herod and his soldiers beat and mock Him, then Herod sends Jesus back to Pilate. Herod and Pilate become friends, but they still have no proof Jesus did anything wrong.