Heijunka is the levelling of production in both volume and product mix over time, so operations run in a steady rhythm rather than in bursts. Instead of producing Monday’s orders on Monday, an average of demand is produced each day.
Uneven load (mura) creates alternating overload and idle time — and forces the organisation to size itself for the peaks. Levelling makes the capacity requirement predictable and enables takt, flow and small batches.
Heijunka presupposes short changeovers (SMED); otherwise frequent switching between variants is too expensive.