What does this quote from Of Mice and Men tell us? "His arms gradually bent at the elbows and his hands closed into fists."

This quote describes Curley's general posture and attitude. He is small and he is totally insecure. He's always chasing his wife around the ranch. This shows that he has little control over her and is easily made jealous. Because of his small stature, he tries to compensate by behaving in an aggressive way. He tries to intimidate others. Shortly after this quote, Curley leaves and the swamper describes his personality to George: 

Curley’s like alot of little guys. He hates big guys. He’s alla time picking scraps with big guys. Kind of like he’s mad at ‘em because he ain’t a big guy. You seen little guys like that, ain’t you? Always scrappy? 

Curley's hands are closed fists because he wants others to think he is always ready and willing to fight. His elbows are bent to indicate he can and will get his arms into a fighting stance. The narrator adds that Curley gives George and Lennie a "pugnacious" glance. This means he is quick to argue or fight. The swamper tells George and Lennie that Curley used to be a lightweight fighter. Because of his insecurities, Curley has always tried to present himself as an aggressive person and someone to be feared. His body language shows this as well.