The theme of a text is what it’s ‘about’, but not the plot. For example, some themes of Othello are jealousy, gender, truth and deception. Themes tend to be abstract nouns, as they’re often ‘big ideas’.
For Lang-Lit, you’ll need to apply this idea to texts other than just literary texts. Themes do emerge in conversations, interviews and media and non-fiction texts as well as literature.