The Road-mender is a country guy who is kind of out of the loop of the Revolution. He is with them in theory, of course, because he, too, is starving. Unfortunately, he has lived far enough away from the aristocracy that the Defarges are afraid that when he sees them he will be mesmerized by their glittery, opulent, and royal selves and therefore be less willing to do what must eventually must be done--overthrow them and kill them. So, they take this little field trip to Versailles to see the King and his royal friends; as expected, the mender of roads is impressed by their finery and has forgotten who has paid for most of it--the citizens, of which he is one. He even cheers for them as wildly as anyone.
The Defarges speak to him as if he were doing so for show, telling him what a grand idea it was to cheer for them so they would not suspect what was coming. Then Madame Defarge uses the metaphors of dolls and birds, with fine clothes and plumage. She asks if he would be willing to strip the clothes and plumage to advance his own cause. Madame Defarge's fatal line is this:
You have seen both dolls and birds to-day.
This is their object lesson, so to speak, to one who needed to both see and understand what he was seeing in order to join the Revolution later.