According to the author of 2 Kings, this verse tells us that there were many other things that King Jehoash did, including his battles with another king, and these stories are written down in another big book called the Chronicles. This helps us know that there's more to King Jehoash's life than what's in this part of the Bible.
2 Kings 14:15: Now the rest of the acts of Jehoash which he did, and his might, and how he fought with Amaziah king of Judah, [are] they not written in the book of the chronicles of the kings of Israel?
This verse lists kings who reigned before any king over Israel, while 2 Kings 14:15 discusses events during the reign of a specific king, conflicting in terms of the timeline and context of kingship.
1 Chronicles 1:43: Now these [are] the kings that reigned in the land of Edom before [any] king reigned over the children of Israel; Bela the son of Beor: and the name of his city [was] Dinhabah.