In a story that's all about sex, it's not too much of a stretch to say the hot poker with which Absolon brands Nicholas's butt is a phallic object, a symbol of a penis. It's even less of a stretch when you consider that the narrator's portrait of Nicholas has feminized him. The moment that Absolon inserts the hot poker in Nicholas's genital region is a culmination of this feminization, with Nicholas playing the female role in a confrontation symbolic of a sexual encounter.
If we're viewing "The Miller's Tale" as a parody of the romance genre, the hot poker could also symbolize a sword (which in medieval romance often symbolized a penis, too). Nicholas and Absolon become two knights, jousting in competition for their lady love.