Deuteronomy 19:4-5

The killer who will live if he flees there is someone who has killed his fellow member of the community by mistake, who did not hate him in the past.

An example would be if a man goes into the forest with his neighbor to cut wood and takes a stroke with the axe to fell a tree, but the head of the axe flies off the handle, hits his neighbor and kills him. Then he is to flee to one of these cities and live there.