In this verse there is mention of a king Herod in rule. This is not the same Herod that was on the throne when Jesus was born. This Herod was the grandson of the infamous Herod the Great. This new king had Jewish blood from his mother's side (Princess Mariamne) so this gave him a more trusted advantage with the Jewish people. When Herod heard of how Peter was socializing with the Gentiles, he took advantage of this opportunity to remain close with his people and had Peter arrested. The apostle James was killed probably during this arrest do to a sword wound.
The king had heard of how Peter had previously gotten out of prison so easily so he ordered 4x the amount of soldiers needed to guard him. Herod was no match to the power of prayer that the church was lifting up. The night before Peter was to be executed an angel appeared to him and helped guide him out of the prison. This being so hard to imagine Peter thought that he was surely having a dream but in fact the Lord had delivered him once again.
A little side note here to think about. If you were going to be executed the next morning would you be able to sleep that night? When the angel found Peter, he not only was asleep but he was asleep on the nasty floor between two powerful men. Peter's faith was so strong that he didn't fear what might happened to him but knew that the Lord's will would prevail.
After being released Peter returned to the apostle's meeting place, the upper room. Once there the people were so shocked that they didn't know how to react. All they knew was that their prayers had been answered. Once Herod found out about the prisoners escape he had all the guards on duty executed even though there was no way that any of them could have stopped the Lord from delivering His follower.
The cities of Tyre and Sidon were located close to Palestine and depended on them for their food supply. But after the two cities had offended the king they were cut off. One day when Herod was about to speak to these people he was struck dead. There is real uncertainty as to why he actually died. Some say that the Lord struck him down because he was considering himself a god because of how he looked. Others think that he was just eaten up with a disease that happened to kill him at that moment. Either his death was just a seal of proof that he wasn't a god but a mere man eaten up with conceit.
Chapter 12: 24-25
Once Herod was dead the persecution of the church came to a stop for the time being. This break gave Barnabas, Saul, and another follower called John Mark a chance to return to the church of Antioch and begin preaparing for the first missionary journey.