This is the rule concerning the man who kills another and flees there to save his life--one who kills his neighbor unintentionally, without malice aforethought.
For instance, a man may go into the forest with his neighbor to cut wood, and as he swings his ax to fell a tree, the head may fly off and hit his neighbor and kill him. That man may flee to one of these cities and save his life.
Otherwise, the avenger of blood1 might pursue him in a rage, overtake him if the distance is too great, and kill him even though he is not deserving of death, since he did it to his neighbor without malice aforethought.
References for Deuteronomy 19:6
This is why I command you to set aside for yourselves three cities.
If the LORD your God enlarges your territory,2 as he promised3 on oath to your forefathers, and gives you the whole land he promised them,
References for Deuteronomy 19:8
because you carefully follow all these laws I command you today--to love the LORD your God and to walk always in his ways4--then you are to set aside three more cities.
References for Deuteronomy 19:9
Do this so that innocent blood5 will not be shed in your land, which the LORD your God is giving you as your inheritance, and so that you will not be guilty of bloodshed.6
References for Deuteronomy 19:10
But if a man hates his neighbor and lies in wait for him, assaults and kills him,7 and then flees to one of these cities,
References for Deuteronomy 19:11
the elders of his town shall send for him, bring him back from the city, and hand him over to the avenger of blood to die.
Show him no pity.8 You must purge from Israel the guilt of shedding innocent blood,9 so that it may go well with you.
References for Deuteronomy 19:13