What does Matthew 14:9 mean?

The king mentioned here is Herod Antipas, son of the man who once tried to kill Jesus (Matthew 2:7, 13). He controls part of Israel under an appointment from Roman authorities. He regrets an impulsive promise he made to a teenaged girl. This girl is his stepdaughter, a child of his wife Herodias by way of Herod's brother, Philip. She has performed a birthday dance for Herod and his guests. Greatly pleased by the dance, Herod has made what was probably a drunken promise to give the girl anything she wants (Matthew 14:1–8; Mark 6:21–23).

Herodias seems only too ready for this moment. She instructs her daughter to ask for the head of John the Baptist on a platter. This is her revenge against the prophet who publicly declared her marriage to Herod unlawful (Matthew 14:4; Mark 6:18). Getting her husband to arrest and imprison John was not enough. She not only wants John dead, she wants his death to be dramatic and humiliating.

Herod's terrible choice is to either dishonor himself in front of his guests by breaking his promise to the girl or to risk civil unrest in his part of the kingdom by killing a man thought to be God's prophet. Herod chooses to make his wife happy. He commands that John be killed.

This party is likely taking place at Machaerus, Herod's fortress east of the Dead Sea. Conveniently, John is also imprisoned at the same location. Herod's order will be obeyed immediately.