Pull means nothing is produced until the next link in the chain demands it — in contrast to push, where production follows a forecast and is pushed onward. Pull limits overproduction and inventory, because consumption controls production.
In practice, pull is often implemented with kanban: a signal (card, empty space, electronic message) tells the preceding link that it may produce or deliver. No signal — no production.
Pull requires relatively stable processes and short changeovers. If changeover time is long, you are forced into large batches, and pull breaks down. That is why pull, SMED and flow are inseparably linked.