A theme is the deeper meaning of a story - it’s what lies at the centre of it, the point of the story.

A story about a shy character that meets a girl but doesn’t know how to approach her, might have an overall theme of overcoming shyness. A theme may be something which helps us, so by reading about it in a story it might help us learn and understand something about ourselves.

Not every story will have a theme, but on the other hand, some stories have many, many themes (for example, any book by Charles Dickens) but if you decide not to have a theme you will need to make up for it with a very strong plot and good writing. Themes make stories more memorable!