What is medieval estate satire?

I moved your question to Chaucer's The Canterbury Tales because that work constitutes one of the best Medieval Estates Satire in any language.

The word estates in that phrase refers essentially to one of three "estates" that medieval people are part of.  Because of the importance of religion and religious leadership in medieval society, the church was the First Estate, and in practice that really meant the clergy--those who (theoretically) comforted and prayed for the general population.  The Second Estate was comprised of the aristocrats, generally those who had direct obligations to the king and owned the land on which common people lived.  The Second Estate also provided the King with his knights and other forces as part of their overall feudal obligations.  Last and unfortunately least is the Third Estate, the peasantry, those at the very bottom rung of the social ladder but who were necessary because they did all the work and provided the food for the other two estates.

Chaucer, as a part of the court, was well versed (no pun intended) in the importance of the Three Estates, and the General Prologue to the Canterbury Tales carries the most concentrated satire of the Three Estates in the entire poem.  Estates satire essentially attempt to point out and satirize the follies and abuses of position that occur in the Three Estates, particularly the first two, and often focused on the church and clergy.  The entire feudal concept of three estates was being modified a bit in the 14thC with the advent of scholars and middle-class (yeomen) people, who were not royalty but did not fit neatly into the peasant class either.  Both of these figure in Chaucer's satire.

Chaucer's satire comes into play, for example, when he describes the Monk who, as part of a monastic order, should be plainly dressed and concentrating on developing his soul, but Chaucer describes him as worldly because he like to dress well, avoid manual labor (a staple of monastic life), and enjoys hunting.  When Chaucer describes another member of the First Estate, the Parson, we see a humble parish priest who is concerned only for his parishioners and practices what he preaches.

The Pardoner, also part of the First Estate, provides Chaucer with a great opportunity for satirizing the church and clergy.  We learn, for example, that the Pardoner sells indulgences to unsuspecting peasants (that is, get out of Hell for free cards), as well as completely fake saints' relics made out of "pigges bones."

Estates satire continued to be a literary motif into the 18thC but saw its height during the 15th and 16th centuries.

What is estate satire?

An estate satire is a work from the Medieval period which attempts to satirize and therefore criticize the social and class systems of the time. The three primary classes of society were the clergy, the nobility, and the peasantry. Most scholars of Medieval society know this hierarchy by the popular wording of "those who pray, those who fight, and those who work." It is important to remember that the shape and form of social hierarchy at the time was thought to be divinely dictated, so it was unwise to directly criticize the arrangement. Estate satire achieved a subtle criticism by displaying classes in their best possible form, so that readers could see the shortcomings of reality.

Chaucer's Canterbury Tales are one of the most well known forms of estate satire, particularly the "Wife of Bath's Tale." While there were women in the clergy, society had been largely cut out for men, and women were a subservient class of their own. "Wife of Bath's Tale" is largely regarded as a criticism of a male dominated society, a common theme in estate satire.

Curiously enough, many of Chaucer's characters do not belong to any of the three estates, but to an emerging middle class that came with the advent of free commerce. Much of estate satire was written during this change, and functions as an attempt to make sense of it.