大难临头 is a four-character idiom (成语) meaning 'a great disaster is about to happen.' It's used to warn that a serious crisis is imminent, often in contexts of urgency or fatalistic resignation. The phrase is built from 大难 (great disaster) + 临头 (to befall someone); it functions as a predicate or predicative phrase, not as a noun. You'll see it in patterns like ∼了 (the disaster is upon us) or 觉得∼ (feel like catastrophe is coming).