Underbrush is the low, tangled growth on a forest floor — bushes, brambles, small trees, saplings — living in the shade beneath the tall trees. It is uncountable: thick underbrush, a lot of underbrush, never 'an underbrush' or 'underbrushes'. The word carries a feeling of difficulty; it is what you have to push through, what hides a snake, what catches fire first in a dry summer, so it turns up constantly in stories about hunting, hiking, escaping, and in news about wildfires, where clearing underbrush is standard fire prevention. It is mainly American; British writers are more likely to say undergrowth, and that is the word to use in exams following British English. Writers sometimes use it figuratively for messy detail you must clear away — the underbrush of regulations — but that is a stretch, not the everyday sense.

林下灌木叢、下層雜木（森林地面上的矮樹叢；英式英語多說 undergrowth）