As a literary device, an allegory is a metaphor in which a character, place or event is used to deliver a broader message about real world issues and occurrences.

Note: Allegory has occurred widely throughout history in all forms of art, largely because it can illustrate complex ideas and concepts in ways that are comprehensible or striking to its listeners, readers or viewers.
Writers or speakers typically use allegories as literary devices or rhetorical devices.

A good example of an allegory is “The lion, The witch and The wardrobe” by C.S. Lewis