"Now it was the custom at the festival to release a prisoner whom the people requested." NIV translation
The "festival" refers to the Passover and Pilate had a tradition of releasing a prisoner that was on the Roman death row in Jerusalem. We remember that he was ever the politician and this act of generosity was intended to maintain peace in the area.
"A man called Barabbas was in prison with the insurrectionists who had committed murder in the uprising." NIV translation
Barabbas was a Zealot that had taken part in one of the many small uprisings against the Roman occupation of Israel. He had been caught and convicted of murder so he would have been sitting in prison waiting for his time of execution. The Zealots opposed paying taxes to a pagan emperor as well as all influence of pagan culture in the land of Israel
"The crowd came up and asked Pilate to do for them what he usually did. 'Do you want me to release to you the king of the Jews?' asked Pilate, knowing it was out of self-interest that the chief priests had handed Jesus over to him. But the chief priests stirred up the crowd to have Pilate release Barabbas instead." NIV translation
Pilate gave the crowd the opportunity to choose Jesus to be released because he knew that Jesus was not really guilty of anything. He knew that it was the religious leaders that wanted Barabbas released and they were wanting Pilate to kill Jesus because they could not.
So, what does this have to do with us as Christians in the world today? This passage shows us the dangers of trusting in any politician as well as trusting in a crowd to do the right thing. Each time there is an election in the world, it seems that politicians try to bribe Christians to make them happy (much like Pilate did with the Jews in this situation). We must remember that a true politician is more concerned about maintaining their own power and position than doing the right thing. We also see that a crowd is easily manipulated into doing the wrong thing. Even so, we have to remember that God was in control of this situation and His plan was still being carried out.