Boxer actually creates two maxims:
"I will work harder." and "Napoleon is always right."
The reader must keep in mind that Boxer represents the hard-working class of Russia. He has bought into the notion that hard work will help the cause of the Motherland and will eventually make life wonderful for all the "commoners."
His second maxim regarding Napoleon better connects to the cancelled debates. He falls into the trap that the debates aren't truly needed because he can believe everything Napoleon says. Orwell uses this to represent the Russian workers slowly giving up more of their rights because they originally trust Stalin completely.
Boxer, the ever self-sacrificing true believer in Animal Farm, decides after Napoleon cancels the Sunday debates that if Comrade Napoleon says something, it must be right. He shortens this into a new maxim:
Napoleon is always right.
This shows the way that, even in the best of people (or in this case animals), relying on an absolute and unthinking slogan can allow tyranny to flourish. Obviously, nobody is always right. Boxer adopts his maxim, however, so that he can cope psychologically with the troubling transformations that are creeping into Animal Farm. His unthinking obedience is dangerous as it enables Napoleon and the pigs to increasingly exploit the rest of the animals. Boxer is a leader, much respected by the other animals for his loyalty, goodness, faith in the revolution, and capacity for hard work. Had he stood up to Napoleon, other animals would have been encouraged by his lead, and Napoleon might have had to stand down or at least would have been slowed down. Orwell's message is that the true believers in a revolution have a responsibility not to blind themselves to reality by refusing to think.