One of the original resolutions put forth by the animal leaders in their new government was that the animals would not engage in trade with neighboring farms. The idea was that the farm would be entirely independent, especially of any form of human help.
By Chapter 6, many of the animals are weak from their food shortagesand disillusioned because of the Snowball incident. So, when Napoleon announces that they will now be trading with the nearby farms, the animals cannot remain silent. Not only do they not understand why they have gone without if the farm actually has food goods to trade, but they are also puzzled by the apparent disregard of a resolution. When they voice their concerns, Squealer--Napoleon's propaganda machine--tells them that there was never any resolution against trade and makes it seem as if their own confusion caused them to think that there was. He even brings in Snowball and tells that he must have misguided them.
The whole incident represents how thoroughly under Napoleon's control the animals are. Propaganda forces them to question their memories of and original goals for the farm and their new way of life, just as Stalin controlled the minds of the Soviet people by rewriting history and incorporating powerful propaganda techniques.