Theme is the foundation of your story. Knowing your theme is essential for successfully constructing your narrative arc, determining the decisions of your characters, and deepening your description. Theme shouldn’t be confused with plot. For example, the book I’m writing at the moment is essentially a mystery about a nanny who goes missing with the two young children in her care. However, the theme of the story considers how people find their way back to one another and themselves after times of struggle and disconnection.
Themes often centre around universally understood concepts such as love, forgiveness, redemption, justice, etc. When you know your theme you can work it into every aspect of your story, and play with it in literary devices, symbolism, characters’ speech, atmosphere, and so on. You can also use motifs, which are recurring narrative threads in a story (whether concepts, symbols or imagery), to reinforce your theme.
If you’re not sure of your theme, ask yourself why you’ve chosen to write this particular story, and look for places in your narrative where you feel a strong emotional connection with the plot, characters or setting. Keep asking yourself questions about why those sections are important to you, and you’ll begin to uncover the underlying theme of your work.