If something is crucial, it is not just important — everything else hangs on it. Crucially is the adverb: you use it to point at that one fact or one step. “Crucially, she had the key” means this is the piece that makes the rest possible. Speakers often put it at the front of a sentence, like a little flag: “Crucially, we must not be late.” It belongs to careful English — news, school writing, meetings — more than to jokes with friends. It does not mean “in a cruel way,” even though the first letters look similar. Though actually, crucial itself comes from an old word for a cross: a crucial moment is like standing at a crossroads, where the next step decides the path. You do not need that history to use the adverb, but it explains why the word feels so strong.