Underlying describes the thing that is really going on beneath the surface — the true cause, the basic principle, the deeper reason — the part that matters most but that you do not notice at first. It always goes before the noun ('the underlying cause', 'an underlying problem'), and it is not used after the verb 'be': you cannot say 'the problem is underlying'. The word contains a small claim that is easy to miss: it says the hidden thing is the important one, and the visible thing is only a symptom. A cough is what you notice; the underlying infection is what the doctor treats. That is why the word is everywhere in medicine ('an underlying condition' — a health problem someone already had), in economics ('underlying inflation' — the trend once you strip out temporary swings), in law, and in any careful analysis. Common partners are worth learning as blocks: underlying cause, underlying problem, underlying issue, underlying assumption, underlying principle, underlying condition, underlying trend, underlying message. In ordinary talk people reach for simpler words — 'the real reason', 'what's actually behind it'. One honest note: it is sometimes used loosely just to mean 'important', with no real sense of hidden depth, and in that use it is close to filler.