If you are talking about a generic definition for theme, it would be "the main point of the story". It's kind of like the moral to the story, or the topic of a talk.
If your teacher is asking you to state the theme of a particular work, be careful about cliche answers like, "All is fair in love and war." Try to use your own words and desribe for the asker what the story was about, what the point of the story was, and what you learned from reading the story.
For example, if I were to give what I thought the theme was for "Brave New World", I might say: "It was a story about a world everyone thought was perfect because they all sought to be happy, but they found out focusing on happiness doesn't make one happy."
Theme is the central purpose of a story. It gives insight about views of life and expresses this purpose through elements in the story. Themes are implie by the author, not stated. When analyzing a theme focus on the central fonflict, nature, and outcome of the story.
When you were young, your parents may have read you stories that ended with “the moral of this story is…”
That moral was a message that you could glean from the overall story. For instance, from The Three Little Pigs we learned about the importance of planning a project well. We know this, because we learned that building a brick house works, while building a straw house is short-sighted.
In a book report, you could state that one theme for The Three Little Pigs is that good planning can lead to success.
But we could also say the theme was about making smart choices.
As you can see, a theme isn’t something that's stated outright; it often appears as a lesson or message that the reader understands by reading between the lines.
A book of fiction can have many themes. Why? Because different people can find different meanings in books.