Earlier in the book, it is established that ribbons or other ornamental cloths are considered clothing, which is prohibited in the rules of Animalism. Mollie, the horse, is upset about this and eventually flees the farm so she will be taken care of and allowed to wear ribbons. In Chapter 9, as the pigs seal their power over the other animals and make themselves exempt from the rules, the ribbon prohibition is oddly overturned:
About this time, too, it was laid down as a rule that when a pig and any other animal met on the path, the other animal must stand aside: and also that all pigs, of whatever degree, were to have the privilege of wearing green ribbons on their tails on Sundays.
(Orwell, Animal Farm, naderlibrary.com)
This is another step in Napoleon's plan for himself and the other pigs to rise to the level of humans, instead of remaining animals. The ribbons foreshadow later events, when the pigs start to wear clothing and walk on their hind legs. By separating themselves from the other animals, the pigs are able to gain power and benefits for themselves while denying them in the name of equality and protection.
The new ruling on ribbons is that they are not allowed anymore. If anyone sees an animal wearing one they will no longer be a comrade. It is ironic because everyone does not wear ribbons except for Molly. They should just later choose if she wants to be a comrade or not. Later on she moves to another farms and starts to wear ribbons again.