George does not want Lennie to talk when they meet the boss because Lennie may say something that may cause them not to get the job.
While they are in the clearing the night before they go to the ranch and their new job, George instructs Lennie to keep quiet when they meet the boss. He tells Lennie that he will give the boss their work tickets, "but you ain't gonna say a word."
George instructs Lennie to just stand there and not say anything when they meet the new boss. "If he finds out what a crazy bastard you are, we won't get no job." However, if they do get on the job, and the boss sees how strong Lennie is and how hard he works before he hears Lennie talk, George feels that they will be "set." That is, the boss will not care that Lennie is slow mentally because he is a virtual work horse.
George has Lennie repeat over and over, "I ain't gonna say nothin'" Also, he has Lennie repeat, "An' you ain't gonna do no bad things like you done in Weed, neither."
"Lennie looked puzzled. 'Like I done in Weed?'"
George cannot believe that Lennie has forgotten. He says that he will not remind him for fear that Lennie will do it again.
Lennie understands and tells George that they were run out of Wood, but George counters, saying they ran on their own: "They was lookin' for us but they didn't catch us." Lennie chuckles and says that he has not forgotten that.
This conversation between Lennie and George hints at what will follow. For, the reader understands that Lennie somehow gets himself into predicaments that cause both men problems.