A theme is an idea that runs through a text. A text [text: Any piece of writing. More widely, a text can be anything that conveys meaning - eg, a film, tv programme, advert, website, or image. ] may have one theme or many. Understanding the themes makes the text more than 'just' a text. It becomes something more significant because we're encouraged to think more deeply about the story - about how it connects to real-life issues, and about what it might mean to us.
The main themes that run through Lord of the Flies are: Things breaking down, War, Violence, Relationships, The Island and Language.
Golding himself wrote of his novel:
The boys try to construct a civilisation on the island; but it breaks down in blood and terror because the boys are suffering from the terrible disease of being human.
The central theme [theme: A central, unifying idea that runs through a text or performance. ] in Lord of the Flies is that of things breaking down. This is shown in a number of ways. Violence replaces peace, friends turn into enemies, life ends in savage death. Everything degenerates.