Theme is the main idea behind a story/poem/song. It is often a universal idea or philosophy. Think of Aesop’s Fables (The Tortoise and the Hare, The Boy Who Cried Wolf). On one level the stories are simple tales that amuse children but they also carry a second, deeper message – a universal truth. This moral is the theme. Such themes are often relevant to everyone, everywhere, in every language, in every culture.
For your writing to be considered ‘art’ you ought to have a theme. Therefore, as well as writing a story whereby Joe wants Natalie, Joe gets Natalie, Joe loses Natalie – you include an underlying message like: “jealousy kills love’.
As you write your story, don’t lose sight of your theme. Some writers use the theme as their title (Jane Austen’s Pride and Prejudice). Though this is not common, nor encouraged, using your theme as a provisional title on your Work In Progress might keep you focused on your message as you are writing. It is also possible that your theme may become a tagline or catchphrase associated with your story, like “Greed is good” for Wallstreet (albeit in contrary form).
Examples: your theme could be a comment on the role that luck plays in a person’s life, or your belief that all beings are interconnected. Moralistic writers might warn against the seven deadly sins: wrath, greed, sloth, pride, lust, envy and gluttony. Alternatively, a writer may want to say such indulgences make the world go around. Whatever your theme is, it is your “message” or “philosophy” and ought to be consistently evident in your story.
Once you have found your theme, a way of reminding the reader of its centrality to your story is to place symbolic “motifs” throughout your work. That is to say, if your theme is jealousy, and a widely known symbol of jealousy is “green eyes” – you could give your character green eyes and/or have him own a green eyed statue that unnerves him. You might also have a lot of “green” in your story. Thus, green becomes your story’s “motif” and will help to create a sense of unity in the piece.