Kanban is a signalling system that controls production and replenishment based on actual consumption: a card, an empty space or an electronic signal grants permission to produce or deliver a fixed quantity. Kanban is the practical mechanics behind the pull principle.
A kanban system puts a ceiling on work in progress (WIP) and makes the flow self-governing: employees don’t need a planner to know what to produce — the signal tells them.
In administrative processes and software development, kanban boards are used to make work visible, limit WIP and find bottlenecks — same principle, different material.