"Napoleon is always right" and "I must work harder" are the two mottos that Boxer adopts in the story.
Those two mottos tell the reader a great deal about Boxer's character. The first motto indicates that Boxer is through and through a blind follower. He is not a leader by nature. Boxer assumes that Napoleon is smarter than him; therefore, Napoleon is always right and should be always followed. Inherent within Boxer's logic is the concept that Napoleon has everybody else's best interest in mind as well. That couldn't be further from the truth. Napoleon cares most about Napoleon and keeping himself in power.
The second motto shows that Boxer is a hard worker. There is nothing wrong with that. A lot of people would do well to have such a motto. Boxer's problem is that he assumes his harder work will always better benefit himself and everybody else equally. That's because he assumes that Napoleon is always right and is looking out for everybody else as well. Boxer does indeed work harder, but his lot in life never improves. His harder work always goes to benefit those chosen few that are in power.