“Kill your babies.”
It’s a rule of storytelling. About the editing process, specifically.
The idea is that, to present the best possible narrative, there are times when the creator will have to cut out their favorite parts. A joke, here. A touchingly dramatic scene, there. If the elements don’t propel the story or characters forward, they must go.
A further, and related, lesson is brevity. The audience should only be given enough information to understand what they are meant to. Any less, and the story will be confusing. Too much, and the story could become boring. Or confusing.
All of this means that the audience should be learning throughout the entire experience. In every moment there should be something new that they didn’t know before.
Ideally, by the end, all questions raised have been answered, the conflict has been resolved, and the characters will have gone through some form of change.
The audience will be left satisfied and, hopefully, wanting just a little bit more.