And among the cities which ye shall give unto the Levites there shall be six cities for refuge, which ye shall appoint for the manslayer, that he may flee thither: and to them ye shall add forty and two cities.
In the cities you give to the Levites, set aside six as safe places for someone who accidentally kills another person. They can run there for safety. Then add 42 more cities for a total of 48 cities.