“A 50-word story is a piece of fiction written in exactly 50 words. That doesn’t mean ‘roughly’ 50 words; it doesn’t mean ‘as close to 50 words as possible’; it doesn’t mean 50 words or fewer. It means exactly 50 words.
As with any other form of fiction, a 50-word story should have a beginning and an end, a plot and character development (even if they are only implied), and a theme, meaning, or purpose of some sort. Many 50-word stories are built around twists or climactic moments.