A statement of theme means that you are being asked to state (write) what a central theme of a piece of literature is. Theme can be defined as a main idea or underlying meaning of a literary work. Generally a statement of theme needs to be followed up with supporting evidence. It's not good form to state that a theme is present without providing evidence to support that claim.
One theme that is present in Twilight (and the rest of the series) is love and desire. Bella and Edward are hopelessly in love with each other. Jacob is in love with Bella. The rest of the Cullen couples are in love with each other. The book is full of teenage love and desire.
I think another theme is the theme of mortality and immortality. That theme is closely tied with the theme of choices as well. Bella is mortal, and the Cullens are immortal. Bella has a choice throughout the series to become like the Cullens, and a great deal of time is spent discussing the positives and negatives of such a choice.