I heard it on a podcast called Writing Excuses. It's more of a feeling thing. You ever feel like, at the end of a movie, things just feel kind of off? It's usually because they messed up their order of things. So like, if little Jimmy is solving a murder mystery, then he gets trapped inside a maze, then he encounters some deeply buried emotional trauma; then he has to resolve those issues in that order. He can't solve the mystery until he leaves the maze, and he can't leave the maze until he confronts his emotional trauma.

Plot threads are like nesting dolls. You can open as many as you want, but you have to close them in the reverse order. Next time your watching a show or movie, keep track of when they open and close plot threads and you'll get it. When it's done right, the audience will feel a good sense of resolution and they won't even know why.