What does 2 Kings 12:2 mean?

Joash began his reign in the seventh year of Jehu's rule, and he was king in Jerusalem for forty years. His mother was Zibiah from Beersheba. 2 Kings 12:2 - Modern Text Bible

(Joash's reign is set in historical context, and his mother's name and origin are given, grounding his story in real people and places.)