It should be noted that the first section was simply to illustrate George Milton and Lennie Small travelling together.
A central idea simply means the main idea that's in a story. The central idea is simply what the author wants the readers to know in the story.
In this case, the message that the author is setting up to communicate to his reader is that George acted as a parent for Lennie and he treated Lennie just like a parent will do for his child.
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