Six of those cities will be cities of safety. If a person {accidentally} kills someone, then that person can run to those towns for safety. In addition to those six cities, you will also give 42 more cities to the Levites.
Any Israelite or any foreigner living among them who killed someone accidentally was allowed to run away to one of those cities of safety. Then the person could be safe there and would not be killed by anyone who was chasing him. The person would be judged by the court in that city.