Napoleon gets the wily lawyer Mr. Whymper to spread propaganda around the local area about how incredibly well the farm is doing under his leadership. It's all a complete lie, of course; life on the farm is characterized by tyranny, bloodshed, and chronic food shortages, but Napoleon wants Whymper to believe that everything's on the up and the up and that the farm has never been more successful.
He wants him to believe this because he's taken the decision to trade with humans in the neighboring farms and villages. If the humans find out about the real conditions on the farm, then they'll try to take advantage of the situation, insisting on paying a lower price for the goods that Napoleon plans to trade with them. They might even go one stage further and use the farm's economic weakness as an excuse to mount a full-scale invasion and ended Napoleon's rule. That's the last thing the power-hungry pig wants, so he's keen to make sure that his false picture of reality is the only one that the outside world will ever get to see.
