Heijunka is the leveling of production by both volume and product mix over a time period. Instead of making 1,000 of product A then 500 of product B in big batches, you produce a repeating mixed sequence (e.g., ABABAB) sized to demand.
Leveling eliminates mura (unevenness), which in turn removes the muri (overburden) and muda (waste) that demand spikes create. It is a prerequisite for stable pull systems and depends on short changeovers.