What does Mark 12:12 mean?

The parable of the tenants is closely related to the metaphoric prophecy in Isaiah 5:1–7. The main difference is that in Isaiah, the vineyard grows only wild grapes, not the domestic grapes the owner had planted. That is, the rebellion against the "landowner" came from the people, themselves. In this parable, Jesus explains that there's nothing wrong with the grapes, it's the tenants, or the religious leaders, who are corrupt and rebellious.

These same men refuse to disavow John the Baptist for fear of the people (Mark 11:32). They argue with Jesus over where His authority comes from (Mark 11:27–33). Then they fail to see the irony in the question. The religious leaders believe their authority comes from God, but they are threatened by Jesus and afraid of the people. That fear leads them to partner with Judas so they can arrest Jesus secretly (Mark 14:1–2). Jesus knows His authority is from God, and He is afraid of no one and nothing, not even death.

In the years since Malachi, the position of high priest had become more politicized. In the 2nd century BC, the role went to whomever bribed Antiochus Epiphanes last. After the Maccabean revolt, the high priest was often whoever had assassinated the previous high priest.

Jesus' point is that it isn't enough to receive authority from God if you don't then use that authority to do His will. The priests, scribes, and elders hold God-given positions, but they don't serve Him. This makes their position very tenuous. If you don't do the will of your master, he won't enforce your authority over the people. That means you have to trick, bribe, cajole, or lord over the people in order to maintain your influence. And if you haven't submitted your will to God, you're going to be more worried about keeping your influence than pleasing God.