When we see crooks, what is he doing to his back?no

Crooks is rubbing salve on his back. He earned the nickname Crooks because a horse kicked him and now he is permanently crooked- hence "Crooks". This seems like a fairly unimportant event in the story, but in fact it is this injury that binds Lennie and Crooks because they both have deficiencies. In a way Crooks' injury comforts Lennie and George because they don't feel like such outsiders at this job, in fact they both think this is a place that they can stay for a while, hopefully long enough to build their stake for their ranch.