Allegory: "an allegory is a narrative in which the agents and action, and sometimes the setting as well, are contrived not only to make sense in themselves, but also to signify a second, correlated order of persons, things, concepts or events. There are two main types: 1) Historical and Political; 2)An allegory of idea and philosophical theories"
--> so, in basic terms, it's like the whole text is a metaphor for something else, but you can understand it/analyse it in terms of the story itself OR the concepts/events the story symbolises (think wind in the willows as an allegory of human society)