When you do something rapidly, you do it very quickly. A rapidly moving train is going at full speed.
You may notice that the grass in your yard grows rapidly during the rainy summer days and more slowly when there's a heat wave. You might also get a speeding ticket for driving too rapidly in a school zone. In either case, something's happening fast. The adverb rapidly comes from the adjective rapid, "moving quickly," with the Latin root rapidus, "hasty, swift, fierce, or impetuous."