Answer:
Squealer can make others believe anything, even lies.
Step-by-step explanation:
In his probably most famous work, George Orwell satirically and metaphorically expressed his views on society, socialism, communism, human nature, lust for power etc.
This particular excerpt describes Squealer as a brilliant and persuasive speaker. Although his body language was also mentioned, the emphasis is on his skill with words, as stated above:
"The others said of Squealer that he could turn black into white.", which pretty much sums up how good speaker he was.