What does Judges 21:9 mean?

In the prior chapter, eleven of the twelve tribes of Israel assembled to bring justice to the twelfth tribe, Benjamin (Judges 20:11–13). That conflict resulted in the near-extinction of the Benjaminites (Judges 20:47–48). To make things worse, those present at the assembly vowed never to give their daughters to men of Benjamin (Judges 21:1). With only six hundred males left alive, the tribe is doomed unless a loophole can be found.

One town of Israel, Jabesh-gilead, sent no men to the assembly. That was the subject of another vow, where Israel promised to kill anyone who failed to support the war. While this means the clan is subject to death, it also means they did not join in the vow about marrying their daughters to Benjaminites. This unique situation is an opportunity for the tribe of Benjamin to avoid dying off.