Something with a specific premise, basically. A character has dinner with a complete stranger. One morning, the sun doesn't rise. The god(s) of a people appear to them. Things like that. The way I'd differentiate from a task-based prompt is that task-based seems to imply something that's more about the format of the story or very small details within the story? So you might ask people to write in second person or include exactly one murder and two teacups.