When you retell a story, you tell it again. You often do not use the same words. You might make it shorter, tell it from another person's side, or tell it in Chinese after you heard it in English. Teachers like this: if you can retell a story, you understood it — you are not just copying the page. You retell a story, a joke, or a piece of news; you do not usually retell one short sentence. Though actually, it is more than “say it again”: repeat can be the same line twice, but retell is a whole new telling of something already told.