A what-if is an imagined situation — something that did not happen but could have, or something that has not happened yet but might. You turn the question "What if…?" into a thing you can talk about, so you can say "a what-if", "the what-ifs", "these what-ifs". It works two ways. Looking backwards, a what-if is a regret or a wondering about the past: what if I had taken that job, what if we had left ten minutes earlier. Looking forwards, it is a possibility you plan around: businesses and engineers run what-if scenarios to check what would happen if sales dropped, or if a machine failed. That second use is quite normal in work English — "what-if analysis", "a what-if question", "let's run a few what-ifs". The plural "what-ifs" often carries a feeling of worry, of a mind that will not stop: "I lay awake with all the what-ifs going round in my head." The hyphen is what turns the question into a noun; you will also see it without one, and as "what if" when it is simply the ordinary question again.