Josiah is only 8, but reigns for 31 years. Not to be confused with the other boy king Joash, under Jehoiada the priest - see 2 Kings 11. Josiah uses the money Joash set aside for temple building repairs. They find a Bible, and it's so unusual that they bring it to the king, who repents and sends to Huldah the prophetess for help. She pronounces doom, but since Josiah's heart is tender to God, it won't be in his reign.
Josiah renews covenant between God and the people. He sweeps Judah clean of idolatry and witchcraft (lots of specifics showing how entrenched and wrapped up with Yahweh temple worship it had gotten!). Even Jeroboam's altar at Bethel comes down, as predicted in 1 Kings 13:2. He restores the Passover, to a height not seen since the Judges. But God's judgement was still coming. Josiah dies in battle with Egypt.
Jehoahaz his son reigns in his place, for 3 months. Egypt imprisons him and makes his half brother Jehoiakim a puppet king, for 11 years. He does evil.
How this is about Jesus
More temple building going on! Jesus built His house with His own blood, and we are its living stones.
Jesus calls for covenant renewal focused on himself, and a clean sweep of idolatry in our hearts (rebuke of Pharisees) and in public worship (temple cleansing).
We get the greatest detail of the exact idolatries that went on in this section. Judah seemed to be hedging their bets, trusting in Baal or Asherah or Milcom, Chemosh, Molech, the sun, etc. There were houses for prostitutes in the temple of Yahweh! We read of some of this in Ezekiel 8.
We must guard our hearts against trusting or seeking satisfaction in other things/beings, instead of the living, one, true God (Deuteronomy 6:4-5).