The order in which the nervous system calls muscle fibers into a contraction, from the smallest and most fatigue-resistant to the largest and most forceful.
This is Henneman's size principle, and it is the single most useful piece of physiology for a trainer, because it explains why effort matters more than load for growth.
Lift something light and only small, slow motor units are recruited. As the set gets harder, either because the weight is heavy or because the light weight has become heavy through fatigue, larger units join in. By the last two or three reps before failure, nearly everything available is working.
That is the argument for taking hypertrophy sets close to failure and for not counting a set of 15 at RPE 6 as training. It is also the argument for heavy work: at 85 percent of 1RM the large units are recruited on rep one, so every rep in the set counts, which is why strength work is more efficient per rep than light work.