If you want to impress your friends and teachers, tell them that "The Miller's Tale" is a fabliau. This was a genre of medieval literature originated by court poet-musicians in southern France. It was concerned with clergy-members and clerks, peasants, and sex. It usually featured someone getting cheated on as a major plot point.
This is obviously applicable to "The Miller's Tale," but in answering "The Knight's Tale" with a fabliau, Chaucer does something really innovative: he shows us how a fabliau can be a parody of the romance genre. With "The Miller's Tale," like "The Knight's Tale," we have a love triangle involving two men and an unobtainable woman – except the love triangle is really a lust triangle, and the woman is unobtainable because she's married! With Absolon, moreover, we have a character who speaks in the high, courtly language of the romance genre, but does so in order to get a girl into bed. This "romance" even ends with a joust of sorts, with a hot poker substituting for a sword. The effect of this parody is to trouble the sharp distinction between fabliau and "high" romance, suggesting that, in both genres, what's really on the table is sex.