This signal triggers production of parts or components to replenish the ones consumed by the next down-stream operation.
When operator removes production Kanban in Op 3 above this triggers the production Kanban cycle.
This Kanban triggers parts to be produced in Op 2.
Kanban is attached to parts in Op 3.
Parts available for material handler to move (as Op 3 (Parts Retrieval Kanban)). This starts process all over again.
There are many other Kanban systems in use, and the following describes two of the simplest.