Whit shows Slim a pulp magazine—specifically, a letter posted to the editor in the magazine. The contents of the letter are pretty banal, simply stating that the writer enjoys the magazine, but Whit is excited about it because he used to work with the man who wrote the letter, who he identifies as Bill. Whit recalls Bill's hopes that his letter would be posted one day.
The fact that Whit is so excited about the letter says a lot. Firstly, it illustrates Whit's connection with Bill. He may not be here physically with him, but Whit still feels a connection with his old coworker. This also emphasizes the loneliness he feels, and that loneliness is juxtaposed with the relationship between Lennie and George.
Secondly, Bill's success in having his letter posted is a major victory in Whit's eyes. In a circumstance where any success beyond surviving from day to day is presented as slim, Bill's modest victory is a cause for celebration.