means The dense layer of shrubs, ferns, and young plants growing beneath the taller trees of a forest.

from A transparent English compound of "under" (Old English under, beneath) and "growth" (from grow, Old English growan, to flourish or sprout). It joins the family of "under-" words — undergrowth, underbrush, undergrowth's botanical cousin understory — all naming the things that live in the shadow of something bigger.