on Writing Forward:

Theme is one of the most difficult story elements to understand. Often confused with plot, theme is actually a worldview, philosophy, message, moral, ethical question, or lesson. However, these labels, taken alone or together, don’t quite explain theme in fiction.

We can think of a theme as an underlying principle or concept, the topic at the center of the story.

Themes are often universal in nature. Some common universal themes are based on motifs of redemption, freedom, equality, sacrifice, betrayal, loyalty, greed, justice, oppression, revenge, and love. Themes can also be personal and part of the human condition. Such themes could explore issues surrounding loneliness, trust, commitment, or family.

However, a story’s theme is more than an idea that can be expressed in a single word. The concept of freedom can form the foundation of a story’s theme, which could be anything from “one should not sacrifice freedom for security” to “freedom is worth dying for.”