In order to make cars efficiently, Toyota uses the "Toyota Production System." The goal of this system is to eliminate all waste so we can deliver cars to customers as quickly and cheaply as possible.
One element of the Toyota Production System is that the necessary parts are made and delivered in the needed amounts at the right times. A special card called a "kanban" is used to make this possible. The "kanban" describes how many parts are used where and when, and is attached to the parts box. When a part is taken from the parts box and used, the "kanban" is removed. The removed "kanbans" are regularly collected from the assembly process and sent to the parts factories. The parts factories in turn produce the number of parts indicated by the "kanbans." In this way, the Kanban System prevents the parts factories from making too many parts and the assembly process from having to allocate space to store parts that are not immediately needed.