A theme, to put it simply, is merely an overarching idea shown, subtly or not, in many scenes throughout the course of the story, and is sometimes embedded in the structure of the plot itself. It ties the story together, but in a different way and on a different level than the sequence of events, the cause and effect, that make up a story. It is often inextricably entwined with not only the basic plot, but also the character's motivations, and even the obstacles thrown at them by the author.

The theme in a story can be very simple - "do your best and things will work out" is a common one, and "as long as Good doesn't give up, it will prevail" is in basically every epic fantasy out there, even the parodies. Even a single, simple theme adds a sense of cohesion to a story. If desired and properly done, a theme can send a message to the reader, made more powerful because it's subconscious.

A story lacking a theme, or lacking a consistent theme from beginning to end, just feels incomplete or unsatisfying. There's a sense of unity that a theme provides that is lacking in a list of events, however beautifully described. This lack is why the story of somebody's day, or week, or life, while it may make an amusing anecdote - even hilarious in the hands of a skilled storyteller - lacks the impact of a story with a theme.

Real life just doesn't have a theme.

But there's more than just identifying a theme in your writing. It has to be consistent, it has to be present - but it's best if it's subtle. A hundred years ago and more, authors could get away with bludgeoning the reader about the head with their theme or moral. It was even acceptable to pull right out of the story and address the reader directly, to tell them about the bludgeoning they'd just received or were about to receive. Fashions change, even in writing, and that style isn't generally acceptable in new writing any more. The theme should peek at the reader from between lines, not run around screaming "look at me! I'm a theme!"

Keeping your themes consistent doesn't mean you have to restrict yourself to one theme, though - it just means that you shouldn't abandon a theme part way through the story, and finish with a completely different one. Weaving multiple themes together, even themes for ideas that can be at odds with each other such as "loyalty" and "independence" can make your story much more interesting. The novel Through Alien Eyes by Amy Thomson, for example, has a strong dose of loyalty to your family, even an adopted, alien family, but at the same time looks at how dependence on others can crush the spirit, and independence and pride is tremendously important. The interwoven themes, even if they're very different, can reinforce each other as well. As in the example above, the two themes together pushed the characters toward strength, self-confidence, and self-reliance, when just one may not have managed the job on its own.

All of these ideas in a story can collide and suggest new ways of looking at things, new events, even new sub-plots for your story. Some of these may be worth working into your story, even at the cost of rewriting chunks of it. With that in mind, review your story every so often to see what themes are cropping up. As mentioned earlier, you may not realize that you're putting a theme in when you write the scene, so a review of the story as a whole may be in order. What you're looking for may not have anything to do with the main plot. In the book Through Alien Eyes mentioned above, the story isn't about either of the two ideas identified as themes, but I think the story would be much weaker without them. Once you've identified the themes that are there, think about the themes and messages you want in your story, if any.

You don't want themes that don't go anywhere cluttering up your story any more than you want to spend two pages describing an item that is never used in the story, so when reviewing your story for themes, you should keep an eye out for those. In most cases, toning down the theme is all that's needed, just as you'd make only passing reference to an item that's there for atmosphere but isn't useful to the story beyond that. In a similar vein, you should think about which themes are more important, and make sure they get more play than those less important. The more time is spent on something, the more important it appears to be in the reader's eyes.

In many ways, theme consistency can be treated in much the same way as you would setting or character consistency. Make sure it doesn't contradict itself, don't emphasize something trivial, don't stop the story to explain something, work it in so you're not beating the reader about the head with it...