Undergrowth is all the low, thick, tangled plant life that grows beneath the big trees in a wood or a forest — bushes, ferns, young saplings, creepers, long grass. It is an uncountable noun: you say 'thick undergrowth', 'through the undergrowth', never 'an undergrowth' or 'undergrowths'. The word carries a feeling as well as a fact. Undergrowth is usually described as something that gets in your way: it is dense, it hides things, you push through it, you lose a ball in it, an animal rustles somewhere inside it. Writers use it constantly for that reason — it is where the unseen thing is. If a forest floor is open and easy to walk on, English does not really call it undergrowth; it becomes undergrowth when it is thick enough to slow you down or hide something. Gardeners and forestry workers use it plainly, without drama, for the layer of growth they need to clear. Note that 'undergrowth' is about the layer as a whole, not one plant — if you want one plant you name it: 'a bush', 'a fern'. A near neighbour is 'underbrush' (more common in American English) and 'brush' for dry scrubby growth.