Definition:
A pull system is a lean manufacturing strategy. In this type of system, components used in the manufacturing process are only replaced once they have been consumed so companies only make enough products to meet customer demand.
A work process where each stage only pulls work into progress when it has the capacity to do so; the opposite of a push system, where work is assigned and added to a queue, regardless of capacity to complete it.
Further Reading:
Book: Kanban: Successful Evolutionary Change for Your Technology Business by David J Anderson
Kanban from the Inside by Mike Burrows