What does 1 Kings 15:27 mean?

Baasha, son of Ahijah from the tribe of Issachar, plotted against Nadab and killed him at Gibbethon, a Philistine town, while Nadab and Israel were attacking it. 1 Kings 15:27 - Modern Text Bible

(Baasha took advantage of a military campaign to assassinate Nadab and seize the throne.)