The central idea in a story is the central element in the story that ties together all of the other elements that are in the story.
The speech isn't given, therefore, an overview of the central idea will be given. The central idea is the dominant impression or the generic truth that's conveyed in a story. It is what the story is mainly about.
It should be noted that the central idea is not the topic of the text or the title of a story. The <em>central idea</em> is usually stated in one sentence. The central idea can usually be seen at the beginning or the ending paragraph.