Imagine your story is a game of Black Jack. Or a Game of Texas Hold'em. The firsts cards are important, it is good to have a nice first draw. But, the most important thing is that, if you decide to play, you have to play with cards you have, and next cards you draw have to work with what you already got.
Lets call each card an "element" of the story. A collection of elements give the story it's tone, it's subject matter, it's genre, it's uniqueness. The elements are the stories DNA. A man is an abstract element. An old man, more specific. The fact that the man is old already limits the possibilities your story may go, it specifies a path. A marriage is an element. A troubled marriage is an specific one.
It's hard to define what constitutes an element. It has to do with specificity and singularity. The more specific and singular, the more elemental.