According to the author of 2 Kings, this verse tells us that after King Ahaz of Judah had been king for twelve years, a new king named Hoshea started to be the ruler in a place called Samaria, which is part of Israel, and he was king for nine years. This helps us understand who was leading God's people and when they were doing it.
2 Kings 17:1: In the twelfth year of Ahaz king of Judah began Hoshea the son of Elah to reign in Samaria over Israel nine years.
Jotham's reign length contradicts the timeline of Hoshea's reign in 2 Kings 17:1.
2 Chronicles 27:1: Jotham [was] twenty and five years old when he began to reign, and he reigned sixteen years in Jerusalem. His mother's name also [was] Jerushah, the daughter of Zadok.