What does Matthew 14:4 mean?

John the Baptist had been arrested by Herod the tetrarch, ruler over the part of Israel that included Galilee and a strip of land along the Jordan River called Perea. Herod the tetrarch, also known as Herod Antipas, had divorced his own wife to marry his brother's wife. John the Baptist had been publicly denouncing Herod and telling him it was not lawful for him to have the woman, called Herodias. For her sake, Herod had John seized, bound, and imprisoned in his fortress (Mark 6:14–19).

Jewish law forbade marrying a brother's wife while he was still living. Herod's action involved more than just breaking Jewish moral law, however. His first wife, the one he divorced, was the daughter of Aretas, the king of the Nabateans. The divorce led to fighting with the Nabateans, forcing the Romans to step in to save Herod. John's public rebuke of Herod likely touched a sore subject. It may not have helped that John spoke forcefully about the coming kingdom of the Messiah, which may have sounded like a threat to the existing authorities in the region.