The presence of the fence around the Radley lot foreshadows the trouble that Jem will have trying to get around the fence.
When the children decide to see Boo Radley for the last time before summer vacation ends (because if he kills them, they’ll miss school rather than vacation), there is a mention of the fence.
We thought it was better to go under the high wire fence at the rear of the Radley lot, we stood less chance of being seen. The fence enclosed a large garden and a narrow wooden outhouse. (Ch. 6)
This fence is foreshadowing, because later Jem will lose his pants and Boo Radley will collect them and mend them, leaving them on the fence. Although it seems like a minor incident, it’s a turning point. It is an example of Boo Radley reaching out to Jem. Jem also demonstrates bravery in going back to get the pants, even though he thinks that Nathan Radley might shoot him. He does not want Atticus to know that he was there. He is not so much worried about being punished as he is Atticus being disappointed in him.
He blew out his breath patiently. "I- it's like this, Scout," he muttered. "Atticus ain't ever whipped me since I can remember. I wanta keep it that way." (Ch. 6)
This incident demonstrates a rift between Jem and Scout. She is too young to see why he did it. She also doesn’t understand what happened with Boo Radley and the pants yet. Jem is getting more mature, and beginning to see how the world works. This is why he understands that Boo put the blanket on Scout’s shoulders at Miss Maudie’s fire. Boo is beginning to reach out to them as friends. He is looking out for them. He sees them as his kids, and cares about them. It is a common thread that will continue throughout the story.
For more information about the characters of To Kill a Mockingbird, watch this video: