These pages appear at the opening of the last section/chapter of the book and they depict Lennie's return to the pastoral and natural environment that he and George had enjoyed in the first section of the story. A number of effects of theme and character are achieved in this passage due to choices of style and content.
First, we can look at the big picture and recognize that Lennie's return to the stream represents the idea that the story (and his character) has completed a cycle. Lennie had been run out of Weed and we now realize that this is probably because he killed a woman there too. He has also been ejected from social life after being tested.
We know he was being tested, in some ways, because of the manner in which George treats him, offering warnings and lessons and always keeping a wary on on him. Lennie does not succeed. This plot point should be seen in the context of a well-crafted and concise narrative pattern as a final repetition of a cycle (test, failure, test, failure). Lennie cannot be successfully socialized in a society where innocence is often punished with cruelty and kindness can become weakness.
He is not a man like other men. This is indicated by the associations that are generated in the text (an aspect of style) as Lennie is characterized by repeated references to animals and is finally situated as being at home in the natural world.
In the opening passages of the book, Lennie is compared to a bear and this reference is made again in the last chapter.
Suddenly Lennie appeared out of the brush, and he moved as silently as a creeping bear moves.
His behavior on these pages is animal-like and he is, again, returned to nature. He had threatened to go off and live in a cave and in these final pages of the novel we see Lennie forced to make his escape and find peace in nature.
His beginning is also his end. A place of innocence that cannot truly remove Lennie from the world, the clearing is a stage (of judgement and of clarity) for both Lennie and George.
Ideas of theme relating to Lennie's ability to live in a social world and of plot structure relating to the cycle of Lennie's tests/failures are presented clearly alongside the subtle alignment of this character with a wild, natural world.
Look for each of these ideas in the text and cite directly from the text as you identify them in the passage.