Boyd’s Bible Dictionary:
The involuntary manslayer found escape in a city of refuge
(Num. 35:22-2322But if he thrust him suddenly without enmity, or have cast upon him any thing without laying of wait, 23Or with any stone, wherewith a man may die, seeing him not, and cast it upon him, that he die, and was not his enemy, neither sought his harm: (Numbers 35:22‑23); Deut. 19:55As when a man goeth into the wood with his neighbor to hew wood, and his hand fetcheth a stroke with the axe to cut down the tree, and the head slippeth from the helve, and lighteth upon his neighbor, that he die; he shall flee unto one of those cities, and live: (Deuteronomy 19:5)).