As writers, it's even more crucial to know what we're trying to say in our stories and essays. Why should the reader care about this? What's it all about?
"Theme is some kind of unifying idea in a story," writes Terry Bain, in her article "Theme: So What's Your Story Really About?" (Gotham Writers Workshop Guide to Writing Fiction) It is a "container to hold all the elements of your story in place."
The best news is that theme needn't be a lofty principle or message - in fact, that's probably the last thing you want. The writer, particularly the fiction writer, shouldn't worry about solving the worlds problems in their tales. It's enough to "shine your flashlight on some aspect of life and let the reader see what's there," Bain continues. "Not every aspect. Some aspect."
For instance, I've been working on a short story about a couple who find their relationship in jeopardy because of the young man's intense attachment to his dead mother's dog. I could have set out to preach about the importance of setting healthy boundaries with parents, or even the necessity of dog training. But, in terms of a basic theme, I've settled on "relationships" as the "container" who focus my story around. Everyone deals with relationships with parents, significant others, and, yes, pets. My story is built around this aspect of life on several levels, so there is something for everyone to relate to.
Bain advises us not to set out writing our stories with theme in mind. Write a draft first, and then study it to see what emerges as a theme. Often, ideas for theme will emerge as you are writing this draft. If not, as you reread it, begin looking at your characters actions, to see whether they imply any universal truths, or whether there is a dominant social context to the story. Try to simplify the story into a few words ~ how would you answer the question "what is your story about?"Once you have determined your theme, you can revise the story to insure that all its elements relate to that general aspect. As you review your plot and characters with theme in mind, you'll be able to see where the theme could be enhanced and find ways to illuminate it. Knowing the theme provides a deeper focus for revision, and helps sharpen your direction for the story.
Best of all, unlike Freshman English class, this time there is no right or wrong answer. Identifying the theme of your story is up to you, the writer, to decide.