What does Acts 25:2 mean?

Porcius Festus, the new governor of Judea, Samaria, Galilee, Syria, and Cilicia, comes to Jerusalem to meet the Jewish leadership. The Sanhedrin immediately asks about Paul.

Two years prior, the Sanhedrin had tried to have Paul prosecuted for inciting riots, leading a cult, and profaning a religious structure. They had no witnesses or evidence—because none of those acts ever happened. Then-governor Felix held Paul in custody as a favor to the Sanhedrin, but nothing else was done (Acts 24).

In Festus, the Sanhedrin sees someone they can take advantage of. He doesn't seem well-informed about Paul's case, so he doesn't know about the lack of evidence. Nor does it seem that he is aware of the assassination plot hatched by the Sanhedrin two years prior (Acts 23:12–15). If they can convince Festus that Paul is a criminal who deserves execution, that solves their problem (Acts 25:24). If they can get Paul out into the open so assassins can kill him, that works, too (Acts 25:3).

"Chief priest" is not a position ordained by God. The qualified male descendants of Aaron were to be priests and the single head was to be the high priest. In the time between the return from the Babylonian exile and the coming of Jesus, the priesthood became entangled in international political intrigue. Sometimes, the priesthood was won by bribing the nearest secular leader who needed money for a military campaign. "Chief priest" became the title of a priest that held a significant amount of power, possibly because he had once been high priest or because his family was powerful. The chief priests and the "principal men of the Jews," including scribes and elders of the city, made up the Sanhedrin.