I first learned about allegories when studying William Blake's poem, 'The Poison Tree'.
An allegory is a story, told in symbolic ideas and metaphors. In other words, saying one thing, but meaning another.
In Blake's poem, the poison tree he grows is a metaphor for his growing anger towards his enemy.
The word allegory is derived from Greek: It's a combination of 'allos', meaning 'other' and 'agora' meaning 'gathering place'.
In times past, people would speak in allegories when they wanted to share secrets or sensitive information in public places.
Today allegories are used as a literary device, to tell a story in a more creative way or to simplify a complex idea.