Squealer is Napoleon's main assistant. His job as the minister of propaganda includes spreading the good word of Napoleon's works. When someone questions his facts, he simply uses lies and threats to make them forget about it. He is constantly yelling out production numbers and other statistics in an attempt to convince the other animals that life on the farm keeps getting better for them. Since they have a relatively short attention span and cannot remember the way things used to be, they take Squealer at his word and he has them convinced. It is because of Squealer's false information that Napoleon is able to take control of the farm in the first place.
Squealer is doing this because he wants to trick the animals into thinking everything is going well. This happens in Chapter 8.
When Squealer does this, things are not good for the animals. They are working really hard and are getting very tired (in part because they aren't eating enough). So the animals are discouraged. In addition, there has been the small rebellion by the hens and a lot of executions in the previous chapter.
Squealer needs to make sure the animals are not thinking about doing any more rebelling or otherwise showing they are unhappy with Napoleon's rule. So he reads them all these statistics that "prove" they are better off than they used to be.