A theme is a one-line explanation of your story... Every story should have one because our stories are about something.
When I say should, I mean that this isn't always the case. Especially so for beginner writers, who may not know, that the theme carries the story. Or even those who've been writing for years may not be aware of the importance of a theme.
And a theme is important.
A theme is what will keep you on track as you write the story.
What happens if you write without a theme in mind? Two things:
1. You stray from the subject
2. You write more than one story.
Let me give you an example of a story written with a theme in mind-
The theme to a story I once wrote was, "Vanity Leads To Destruction."
Very briefly, this story is about a female character that believed herself to be so beautiful, in the end she lost everything.
Writing this theme on a post-it note and sticking it on my computer, I was always reminded that my story had to revolve around this theme. Having a theme helped me stick to the story I had intended on writing and not stray from it -- not even a little. It also helped me to focus only on what was essential to my story.
So according to my theme, "Vanity Leads To Destruction":
- Every action my character performed was to show her vanity
- Her goals sprung from her vanity
- What motivated her was her vanity
- Her words (dialogue) showed her vanity
- How she handled situations showed her vanity
- The interaction with other characters showed her vanity
- The conflict was a result of her vanity
- The highest peak in my story, the climax, showed an intense moment of whether her goal would be met (whether her vanity would work for, or against her)
- And the ending? The ending showed how she was led to destruction because of her vanity
By following my theme, everything in my story was precise.
If I showed my character not being vain in any instance, then I would've strayed from my theme.
Does your story have a theme?
If you didn't consciously write with a theme in mind, then your story won't have one. Chances are, your story isn't focused. But then again there is the other possibility- At times we fluke things.
Perhaps without consciously knowing, a theme is running through your story.
Check your story to see if this is the case. If it is, are you sticking to the theme?
If you find that some parts of your story are straying from the theme, those parts will need to be changed to accommodate the theme.
Now the other possibility:
You don't have a theme and none is running through your story. That's okay. We can still save it. Read through your story and see what it's about. Then come up with the most appropriate theme to it -- a theme that makes sense yet won't mean too much work to change the story.
Rewrite those parts that don't accommodate the theme.
It is easier if we come up with a theme while we are at the plotting stage. It saves all that rewriting but not to worry -- you won't make the same error again in your next story!