Flow means an item or a case moves through the process without stops, accumulation or waiting. Perfect flow is one-piece production: each item is processed and passed on immediately.
Flow reveals problems that batches conceal. When items accumulate between processes, the piles hide imbalance, breakdowns and quality defects — and errors are discovered long after they arose. The closer to flow, the faster the feedback and the shorter the lead time.
Full flow is not always possible or economical. The goal is to maximise flow where it can be done, and couple the processes with small, controlled buffers (supermarkets/kanban) where it cannot.