What does Mark 6:21 mean?

Herod Antipas is one of four rulers of Israel. His domain includes two areas: Galilee, between the Mediterranean to the west and the Sea of Galilee and part of the Jordan River to the east; and Perea, east of Judea across the Dead Sea and the Jordan River. Apparently his birthday dinner is held in Perea, close to the prison where John the Baptist is held. It's unclear who the leading men of Galilee are. Most Jews had reason to despise Antipas. He is part Samaritan and part Edomite, both of which are ethnicities which most Jews disdained. He rules by order of the Roman oppressors, not a legitimate kingship. And he reportedly built his capital, Tiberias, on an ancient burial ground. If a Jew comes into the city, they will be unclean, and to live in the city is untenable.

The Herodians, however, value political power more than the Mosaic Law, and have no problem supporting Antipas. In this, they are the opposites of the Pharisees who impel the people to obey not only the law, but the oral traditions. The only thing the two groups have in common is their hatred of Jesus, and after Jesus heals a man on the Sabbath they join together to destroy Him (Mark 3:1–6). The only other mention of Herodians in the Bible is when they join the Pharisees to try to trap Jesus. They ask if it is lawful to pay taxes to Caesar, which the Herodians would be fine with, or not, which the Pharisees would prefer. Jesus responds, "…render to Caesar the things that are Caesar's, and to God the things that are God's," (Mark 12:17) effectively putting both sides in their place.