A Pull System is a type of production system used in Lean Production in which production is triggered by demand. Usually when an item, or small batch of items, is 'consumed' a kanban triggers replenishment. Kanbans 'pull' product through each stage of production.
This differs from a traditional 'push' system where production is scheduled to meet anticipated demand, and then 'pushed' through production as quickly as possible. Push systems usually generate too much inventory and items that should have priority are often delayed behind items that are less urgent.