Instructive means "useful and informative." Say you're visiting Paris and looking for the Eiffel Tower when a friendly Parisian native helpfully stops to draw you a map. "Merci," you say. "This map is very instructive."
In English, anything that informs or enlightens is instructive, but in the Finnish language there's something called the instructive case, which is a whole separate way of saying "by the means of." An example would be a sentence like "I traveled here by horse," which you would say differently than any other kind of sentence you might utter. You would also be speaking Finnish, so the information would only be instructive to Finns.