Today I thought we'd talk about theme. You may have hazy (and possibly bad!) memories of the subject from your English Literature lessons at school, but paying attention to theme can prove an enriching experience for your manuscript.
Theme refers to the ideas that a book explores underneath the story itself.
Some broad examples include love, loss or redemption. The central theme in Harry Potter, for example, is the battle of good versus evil (with the conclusion that good will ultimately triumph). There are many other themes, though, including an exploration of prejudice (between muggles and muggle-born magic folk and 'pure bloods') and the idea that even the smallest or least confident of us (Dobby and Neville) can play vital and heroic roles.
Theme is different from plot, character, setting, and language, but it comes from all of them.
The word order in that sentence is important: theme comes from the story, not the other way around.
If you start with a theme or, worse yet, a lesson you want to teach, then you will be bending your story to fit that, rather than letting it grow organically. Plus, the story is the most important thing – don't downgrade it in the pursuit of didacticism.
Once you have a completed draft, you will be able to identify the themes of your story and, if you wish, add imagery or action to strengthen them. You can also prune away things which don't add to your themes (or which counteract them), helping to illuminate and sharpen the things you wish your book to 'say'.
Repeating imagery or motifs can be used to highlight your themes and to add depth and interest to your story.
All of which is not to say that you must do this. You can, if you wish, ignore the concept of 'theme' completely and concentrate purely on plot, character, story and emotional depth. Rest assured, the themes will be there, whether you intend them to be or not.