prequel; retrospective talk
Refers to a story or discussion that covers events leading up to a main story or current situation. Often used in media contexts (films, novels, games) for a prequel, but can also mean a retrospective account or talk about past events.
A prequel to this movie was published as a novel.
At the beginning of the meeting, I explained the background as a retrospective talk.
後日談 refers to an epilogue or story about what happened after the main events, whereas 前日談 covers events leading up to the main story.
Compound of 前日 (zenjitsu, 'previous day') and 談 (dan, 'talk, story'). The exact origin is uncertain, but it follows the pattern of similar compounds like 後日談 (gojitsudan, 'epilogue').