IPA: /ˈʌndəɡrəʊθ/
KK: /ˈʌndərˌɡroʊθ/
Low-growing plants and small trees that grow under larger trees in a forest.
The deer moved quietly through the thick undergrowth, searching for food.
Undergrowth is formed from "under-" (meaning beneath) and "growth" (from Old English "growan", meaning to grow). The word refers to the vegetation that grows beneath the taller plants in a forest or wooded area.
Imagine plants that are 'growing' ('growth') 'beneath' ('under-') the taller trees — that's what undergrowth means.