A theme is a subject or idea that keeps appearing in the text. You usually find more than one theme in a story. They help to hold the story together, and often encourage us to think more about the plot, and what lies beyond it
"You're so innocent, you didn't realise. You're too trusting, too kind, you don't know what these people can be like."
What is interesting about these contrasting ideas is that we realise things are not always what they seem. The narrator seems to have high moral standards, but when she was young she broke the rules, too.