Specifically, he's using indirect characterization to reveal Lennie's personality. We learn about Lennie's love for soft things through his actions (touching them) and his thoughts (thinking about how soft they are).
Here's why it's indirect characterization:
* It doesn't directly tell us what Lennie is like. We don't have a narrator explicitly saying "Lennie loves soft things."
* We infer Lennie's character traits based on his behavior. His desire to touch soft things suggests that he is gentle, perhaps naive, and possibly has a childlike fascination with the world.
This early glimpse into Lennie's personality helps us understand his motivations and sets the stage for the tragic events that unfold in the novel.