What's a frame story?

(I'm doing ancient literature right now and I don't understand what this is)

A frame story is a literary technique that serves as a companion piece to a story within a story, where an introductory or main narrative sets the stage either for a more emphasized second narrative or for a set of shorter stories. The frame story leads readers from a first story into one or more other stories within it. The frame story may also be used to inform readers about aspects of the secondary narrative (s) that may otherwise be hard to understand. This should not be confused with narrative structure or character personality change. A frame story is also called a "sandwich narrative" or an "intercalated narrative." One narrative is embedded or nestled within a second story and acts as a commentary on the frame narrative or vice versa. This framing device is a common literary technique within the New Testament. For example, Mark interrupts the story of the cursing and withering of the fig tree to place another story within it. The fig tree narrative acts as a commentary on the Temple cleansing.