John 19:16 meaning

The parallel Gospel accounts of this event are Matthew 27:26, Mark 15:15 and Luke 23:24-25.

Having been cornered by the chief priests' blasphemous response, "We have no King but Caesar" (John 19:15), Pilate buckled to the shrewd political maneuvering of the Jewish leaders and granted the crowd's demand that Jesus be crucified. Pilate had wanted to release Jesus (Acts 3:13b) but felt that he could not do so without provoking a riot. If a riot ensued, he might lose his position, so he chose "to satisfy the crowd" (Mark 15:15a), 

"When Pilate saw that he was accomplishing nothing, but rather that a riot was starting, he took water and washed his hands in front of the crowd, saying, 'I am innocent of this Man's blood; see to that yourselves.'"
(Matthew 27:24)

Pilate's washing of his hands was either a sixth and final attempt to release Jesus, or it was an effort to pre-emptively absolve himself from the crime he was about to commit when he handed Jesus over—or some form of both.

To learn more about the significance of Pilate's handwashing and the bloody response from the crowds, see The Bible Says commentary for Matthew 27:24-25.

The crowd's enthusiasm that he was about to grant their demand reassured the troubled governor. 

"And all the people said, 'His blood shall be on us and on our children!'"
(Matthew 27:25)

Within forty years their deadly oath would be due. Rome destroyed Jerusalem, and demolished the Temple in 70 A.D. According to the ancient historian, Josephus, who was a witness to Jerusalem's fall, when the temple was destroyed by fire, he writes that one would have thought that the temple mount itself was ablaze because it was "full of fire on every part of it," but "the blood was larger in quantity than the fire" (Josephus. "Wars of the Jews". VI.271) Because the siege of Jerusalem took place during the Passover, Josephus estimated that 1.1 million Jews perished and another 97 thousand were taken captive (Josephus. "Wars of the Jews". VI.420). 

The Jews "were insistent, with loud voices asking that He be crucified. And their voices began to prevail" (Luke 23:23). 

So he (Pilate) then handed Him (Jesus) over to them to be crucified (v 16). Pilate, the governor tasked to keep Roman law, broke that law and crucified a man he had declared innocent in order to keep his political position. The Jewish priests who were sworn to serve no King but God committed blasphemy, declaring Caesar to be their king, in order to maintain their status (John 19:15). 

Roman crucifixion entailed fastening its victims to a raised wooden beam by the wrists (often with nails) and hang them there until they died. Archeological evidence also shows that the ankles were sometimes nailed, not together on the front of the cross, but separately on either side of its main beam. Crucifixions were done in public places with the crimes posted for everyone to see as a way to be deter future transgressions. Criminals would sometimes agonize on their crosses for days before they perished from suffocation, dehydration, or cardiac arrest. The entire process was devised to be torturous and humiliating and serve as a deterrent for breaking Roman law.

To learn more about this brutal form of execution, see The Bible Says article, "Bearing the Cross: Exploring the Unimaginable Suffering of Crucifixion."

And so, after many diabolical twists and unexpected turns, the religious leaders' conspiracy to murder Jesus had achieved its goal. What was put in motion when Judas alarmed the priests that Jesus had identified him as part of the plot twelve hours earlier, led to their scrambling to arrest Him; secretly and illegally gather for nighttime trials; flail to find a charge; stage a "crime" for which to condemn Him; retry Him at sunrise; bring Him before Pilate; follow Him to Herod's court; and relentlessly pressure Pilate to crucify Him—had resulted in the outcome they sought. Jesus was put to death so that the priests and elders could keep their position within the nation cJohn 11:49-50).

Nothing about this outcome surprised Jesus. 

He had predicted that He would be handed over and crucified (Matthew 20:18-19). 

None of this surprised God. 

God foreknew, even before the world was created that this would happen (Revelation 13:8). God predicted this when Adam and Eve sinned (Genesis 3:15). Isaiah makes it abundantly clear that it was the LORD's plan, will, and desire that Jesus be killed:

"But the LORD was pleased
To crush Him."
(Isaiah 53:10) 

"The things which God announced beforehand by the mouth of all the prophets, that His Christ would suffer, He has thus fulfilled."
(Acts 3:18)

God sent His Son into the world so He would die for the sins of the world (John 3:16).

With Pilate's handing Him over to be crucified, the third and final phase of Jesus's civil trial had concluded. 

The three phases of Jesus's civil trial were:

1. Jesus's Arraignment before Pilate (Matthew 27:1-2, 11-14, Mark 15:1-5, Luke 23:1-7, John 18:28-38)
2. Jesus's Audience before Herod Antipas (Luke 23:8-12)
3. Pilate's Judgment (Matthew 27:15-26, Mark 15:6-15, Luke 23:13-25, John 18:38 - 19:16)

The third phase of Jesus's civil trial was at the Praetorium, Pilate's Jerusalem headquarters (John 18:28, 19:9). This phase began while it still morning, most likely sometime around 8:00 a.m. and likely concluded sometime around 8:30 a.m. (According to Mark 15:24, Jesus was hung on the cross at 9:00 a.m.). Based on the Jewish calendar, the date was likely Nisan 15—the first day of Unleavened Bread. By the Roman calendar, the day was a Friday.

To learn more about the timing and sequencing of these events see the Bible Says, "TIMELINE: JESUS'S FINAL 24 HOURS".