One must never place a loaded rifle on the stage if it isn't going to go off. It's wrong to make promises you don't mean to keep.
– Anton Chekhov
Whenever I watch movies, I always pay attention to otherwise unrelated elements that seem to get a little extra screen time or one too many mentions. How could those elements foreshadow the plot? Chekhov's gun is a principle that says that every component of a story must have a reason, and irrelevant elements should be removed. Often you can predict what's going to happen by looking for Chekhov's Gun.
In product development, the minimum viable product says that you should build just enough features so that it is usable by early customers. Chekhov's gun is an extension of the MVP. So keep the feature set focused. The more predictable the user journey is, the better—no unnecessary details.