A lean supermarket is a small, controlled inventory of finished or in-process items, held in fixed locations and quantities, from which a downstream "customer" process withdraws what it needs. Withdrawal triggers a kanban to the upstream process to replenish exactly that amount.
Supermarkets are used where true continuous flow isn't feasible — they decouple processes while still enforcing pull and capping inventory. They are a central element of value-stream design.