God designated six 'cities of refuge' for the children of Israel (Joshua 20:7, Deuteronomy 4:41-43). Three of these were on the west side of the Jordan river and three others were on the east side. They were strategically placed so that anyone in Israel could flee to a city of refuge in a day or less. These Levitical cities were specially chosen as places where those who unintentionally killed a person (involuntary homicide) could flee to and be protected from the possibility of revenge.
Once they arrived in the refuge city those who accidently killed someone were to seek out the leaders in the area and explain to them what happened. The city's leadership was then responsible to give the person a place to live. They were also strictly forbidden from handing the person over to anyone who wanted to exact revenge over the unfortunate death. God then laid out the conditions for the Israelites by which the person who (not in anger) killed another human could leave the city.
6 You may stay in the city until you have received a public trial and until the death of the man who is then the High Priest. Then you may go back home to your own town, from which you had run away.' (Joshua 20)