George thanks Slim for what he had done for Lennie. Slim gave Lennie a puppy. This is an important part of the story for a few reasons.
First, there is an example of kindness. By giving Lennie a puppy, Slim shows generosity and kindness, something that is missing in the world of migrant workers. From this perspective, this kindness is a glimmer of hope. Therefore, Slim's action is no small matter.
Second, from the beginning of the novella, it is clear that Lennie loves soft things. In fact, in the first chapter, we see that often Lennie picks up mice to pet. For some reason Lennie's favorite thing to do is touch soft things. Now he has a dog; Lennie's heart is content.
Here is the conversation between George and Slim:
“It wasn’t nothing,” said Slim. “I would of had to drowned most of ‘em anyways. No need to thank me about that."
George said, “It wasn’t much to you, maybe, but it was a hell of alot to him. Jesus Christ, I don’t know how we’re gonna get him to sleep in here. He’ll want to sleep right out in the barn with ‘em. We’ll have trouble keepin’ him from getting right in the box with them pups.”