In this chapter, Jehoshaphat appoints religious judges over Judah.
This chapter has two main parts.
The first part (in verses 1-3) is a continuation of the previous chapter. After being defeated by the Arameans, Jehoshaphat has come home to a complaining prophet. Much like a grumpy wife after her husband has a late night out with the boys, Jehu seems to be waiting around for Jehoshaphat to get back home to Jerusalem and then chews him out. Of course, we can imagine Jehoshaphat trying to defend himself, claiming it was all innocent fun and that "nothing happened", but prophets usually have an intuition for this sort of thing and Jehu calls him out for his alliance with Ahab. For all of the reasons that I discussed in the previous chapter, this alliance with Ahab was a mistake. By teaming up with Ahab, Jehoshaphat is only "bringing wrath on himself from the LORD" (v. 2). However, the prophet also acknowledges that while Jehoshaphat is making mistakes, he has "set his heart to seek God" (v. 3).
Jehoshaphat is a man with divided motives. On the one hand, he is dedicated to God and institutes religious reforms to bring the nation closer to God. On the other hand, Jehoshaphat's political and pragmatic side seems to be drawn towards allying with the northern kingdom, in spite of their major religious and cultural differences, and it's this pragmatic side that seems to be working to Jehoshaphat's detriment. While these two sides are conflicted, it appears that Jehoshaphat's devotion to God is enough to get him through the problems he creates for himself, much like how his "crying out" (2 Chron 18:31) to the LORD in the previous chapter was enough to save his life when enemies were chasing him.
In this chapter, Jehoshaphat once again proves his merit because immediately after being rebuked, he goes out to bring his people back to the LORD (v. 4). You can contrast his response to a critical prophet to Asa's response in 2 Chronicles 16. In that case Asa, who was previously a godly king, is enraged at the criticism and throws the prophet into prison for speaking against him. Jehoshaphat responds with much more humility and rather than lashing out against the prophet, Jehoshaphat responds by seeking God in an even more dedicated way than before.
That brings me to the second part of this chapter, Jehoshaphat's religious reforms. As part of bringing the people back to the LORD, Jehoshaphat appoints judges in the towns of Judah and places Levites and priests as judges and officers over the other judges. This is not the first time that Levites or priests were appointed as judges, or at least suggested as judges. Deut 17:8-13 says that "difficult cases" should be brought to the temple, to the "Levitical priest" to adjudicate the issue. It's possible that Levites-as-judges was previously commanded, but is only now being fulfilled in actual fact.
The practical implication is that Levites and priests would have a greater role in society, and would have more leverage for enforcing the religious precepts of the Law of Moses. It suggests a gradual shift away from the traditional forms of law which was judged by tribal elders and towards the religious form of law that is contained in the Torah, the Law of Moses. The Law of Moses has been around for a long time, but this is a practical step towards actually implementing it in Judah's society.
Jehoshaphat also divides the administration between affairs of the LORD (religious law) and affairs of the king (royal law). This is a fairly logical division, since the high priest would be most well-versed with priestly law and the eldest leader of the house of Judah would be familiar with the king's interests and would be capable of advocating on the king's behalf.
All of this is positive news for Judah. It looks like they have a godly king and are slowly starting to align with the Law of Moses. In the next chapter, Jehoshaphat faces his second major test and his first major invasion as Judah is attacked by their enemies.