The culmination of a civil war between the Aws and the Khazraj of Yathrib that had lasted over a decade, fought over land, watering, and grazing rights. The battle killed the bulk of the senior leaders of both tribes, leaving mostly a younger generation weary of a decade of bloodshed. Abdullah ibn Ubayy ibn Salul was the one senior leader left standing after the battle.
By decimating the old leadership of the Aws and Khazraj, Bu'ath left the younger survivors receptive to something new, clearing the way for the reception of Islam in Yathrib soon after.
Other events recorded around the same time. Their order within the year is not established; use the storyline above for the sequence.
The Day of Bu'ath was a gift from Allah to the Prophet ﷺ. It was because of Bu'ath that their (the Aws and Khazraj's) unity had been fragmented and their leaders completely decimated.
A later reflection on the significance of the battle.