A Johnny-on-the-spot is a person who is right there when something needs doing — they offer help at once, or they grab a chance the moment it appears. You might say you needed an extra driver, and luckily someone was Johnny-on-the-spot. It is informal, a bit old-fashioned, and mostly American; younger people may not say it much, though they will still understand it in a story. Johnny here is not a real person's name. English has long used Johnny as a stand-in for some fellow, and on the spot means both in that place and at that very moment. Put together, it paints a person who turns up exactly where and when they are useful. It can sound warm (what a help) or a little teasing (always leaping in). You will also see it written with different hyphens, or with none.