快要 means 'about to' or 'almost (doing something)' — it expresses imminent action or a state that is on the verge of happening. The character 快 literally means 'fast' or 'soon', and 要 means 'to be going to' or 'will'. Together, they intensify the sense of immediacy: not just 'will happen', but 'is on the very brink of happening'. It’s commonly used before verbs (e.g., 快要下雨了 — 'It’s about to rain') and often appears with aspect particles like 了 to signal change of state.

This phrase conveys urgency or nearness in time — more emphatic than just 要, and less formal than 将要. It’s frequent in spoken Mandarin and everyday writing, especially when describing weather changes, emotional shifts, deadlines, or physical sensations (e.g., exhaustion, hunger). Note that 快要 cannot stand alone as a predicate; it must modify a verb or verb phrase and usually requires context indicating temporal proximity.