All Israelites or any foreigners living among them who killed someone by accident were allowed to run away to one of these cities of safety. They could be safe there and not be killed by anyone chasing them. The court in that city would judge them.
Six of those cities will be cities of safety. If a person accidentally kills someone, 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.