This is Failbetter’s term for using underspecified narratives for effect. The idea is that the storylets a creator writes are the fires. But the paths between those stories are dark – they’re the player’s own.
What we mean by this is that you should let players fill in the blanks between your stories. If your story says ‘I was away for two weeks, and when I returned, I saw…’, there’s no need to specify what the character was doing in the time away. If they’re engaged, the player will most likely fill in the details with something that makes sense to them. Or if you describe a character saying, ‘Haven’t we met before?’ the player might mentally fill in whether that character is remembering correctly or mistaken. So, there’s no need to fill in every detail. The player’s imagination is your ally.