Which satirical technique is used in this quote from George Orwell's Animal Farm?''The animals were not certain what the word meant... they...

In order to best answer the question, one first needs to understand the concept of irony. It is a method which an author uses to expose and criticize, make fun of or ridicule a person, situation or thing by pointing out its inherent or acquired flaws or shortcomings. 

This particular quote is an extract from the final paragraph of Chapter Five, after Snowball had been aggressively expelled from the farm by Napoleon's vicious dogs. He had, at the time, been speaking about the benefits of building a windmill and it was pertinently clear that the majority of animals would vote for him. Napoleon recognized the threat and got rid of him.

Later, Napoleon said that the windmill had been his idea from the start. When Squealer, who had been going around explaining this new development, was asked why Napoleon had been opposed to the idea in the first place, he slyly said that this was a 'tactic.' This is the word that the animals could not understand but they accepted Squealer's explanation anyway. The passage reads, in part: 

...He had SEEMED to oppose the windmill, simply as a manoeuvre to get rid of Snowball, who was a dangerous character and a bad influence. Now that Snowball was out of the way, the plan could go forward without his interference. This, said Squealer, was something called tactics. He repeated a number of times, "Tactics, comrades, tactics!" skipping round and whisking his tail with a merry laugh.

The satire is evident. Orwell is using two forms of satire in this instance. Firstly, he is using verbal irony by exaggerating the animals' stupidity. The extract illustrates most pertinently how easily the animals could be, and were, manipulated and deceived because they lacked the intelligence to understand what was being said. As a result, they accepted what they were told. This gave the pigs an even greater advantage to then abuse them since they could merely use big words to further confuse them and, in the process, get their way. 

A similar incident in which this is illustrated is when Snowball spoke about wings as a method of propulsion in trying to explain to the birds why they could be categorized as animals since they had only two legs. The extract is from Chapter 2: 

The birds did not understand Snowball's long words, but they accepted his explanation, and all the humbler animals set to work to learn the new maxim by heart.

The animals' easy acquiescence to the pigs' authority is also ridiculed in the form of dramatic irony. We, the readers, know what Squealer is speaking about and are humored by the fact that Squealer is also using a 'tactic' to manipulate the animals. His particular enjoyment at using the word makes it clear that he derives much pleasure from the realization that the stupid animals do not have the faintest idea that he, at that very moment, in which he is speaking about the 'tactics' that Napoleon used against Snowball, is also using a tactic to mislead them.    

In the end, the other animals are thoroughly confused and intimidated by the pigs and hardly know whether they are coming or going. This situation creates the perfect conditions for the pigs' avarice, arrogance, and malice to flourish.