Albert Barnes Commentary Numbers 35:12

"And the cities shall be unto you for refuge from the avenger, that the manslayer die not, until he stand before the congregation for judgment." — Numbers 35:12 (ASV)

The avenger - Hebrew גאל gā'al — a term whose original meaning is uncertain. The very obscurity of its etymology testifies to the antiquity of the office it denotes. That office rested on the principle of Genesis 9:6, whoso sheddeth man’s blood, by man shall his blood be shed. The unwritten code of the East conceded to the nearest kinsman of a murdered man the right of avenging the blood that had been shed.

Such primitive justice necessarily involved grave evils. It gave no opportunity for the person charged with the crime to establish his innocence; it recognized no distinction between murder, manslaughter, and accidental homicide; and it perpetuated family blood-feuds, as the avenger of blood was liable to be treated in turn as a murderer by the kinsman of the man whom he had killed.

These grievances could not be removed as long as there was no central government, but they could be mitigated; and to do this was the purpose of the institution in the text .

Among the Arab tribes, who are under the control of no central authority, the practice of blood-revenge remains in full force to the present day.

The congregation - that is, the local court, consisting of the elders of the city (Joshua 20:4).