What is the platform (the conceit that allows the story to exist)?

To platform is to allow a story to be told by giving it a reason to be. I've been using the word "platform" for this idea, though I know others use different terms.<br><br>For example, if you have a zoomer main character on a space station, they're meeting aliens right and left. It would be silly for them to be constantly explaining what the aliens look and act like. But slam them into a medieval village in Tuscany, and you have an excuse for a narrator to explain what aliens are like. <br><br>Or, say you want to explain the intricacies of your magic system, where better to do it than at a school for young mages? And it's the same for a civics lesson, why not at a school for young people.<br><br>You get the idea.<br><br>What's your favourite platform for a work of fiction?