Piers Plowman is incredibly rich in allegory and allusion. One of the more famous of these is the parliament of rats, which begins:
Then began all the commons · to cry out in Latin,
For counsel of the king · construe how-so he would:
`Praecepta regis sunt nobis vincula legis.' ["All that the king ordains binds us with chains"]
With that there ran a rout of rats at once,
And small mice with them · more than thousand,
And came to a council ·for their common profit....
Piers Plowman is one of the great examples of literature that requires some historical context. In 1215, the first edition of the Magna Carta was written following an uprising against King John. However, the Magna Carta went through various editions, and did not become statute law until around the time of Piers Plowman in the end of the 14hth century.
England was in some political turmoil at this time, and Piers Plowman becomes a vehicle for expressing displeasure with the government in allegorical tones. Here's an excellent article on this phenomenon:
Can you find any other political or social allegories in the poem?