The fence separating Bruno's family's house from Auschwitz might seem like a pretty straightforward symbol: It's a fence, so it keeps things apart. Next, please.
But not so fast. Let's take a look at this fence, shall we? We're told:
There was a huge wire fence that ran along the length of the house […] the fence was very high, higher even than the house they were standing in […]. (4.162)
A picket fence this most definitely is not—if anything, it seems more like a cage; it's that all encompassing and inescapable. Which, of course, is its purpose: keeping the prisoners of Auschwitz right where the Nazis wants them—away from everyone else, and ready to do whatever it is the Nazis desire at the drop of a hat.The fence, then, represents the terrible work of the Holocaust.
The fence isn't just about what's on the other side, though; it's also about the side Bruno lives on with his family. On their side, nobody openly questions what happens within the camp, and instead Bruno and the rest of his family basically just keep on keeping on, despite the plight of their neighbors. In this way, the fence draws our attention to the sharp contrast between the lives unfolding on each side of the fence—one filled with freedom and plenty, and the other destroying people one day at a time.
While people like Bruno's father and Gretel are perfectly content on their side of the fence—making it clear they see Jewish people as different from them (otherwise they would not be cool with the fence)—when Bruno meets Shmuel, he'd love to cross over to play with him. But he can't. The fence keeps the boys apart for the vast majority of their friendship, making it clear to readers that forming friendships across ethno-religious lines under Nazi rule is no easy feat.
In fact, to say it's no easy feat is really an understatement: When Bruno does cross the fence, disguising himself as a camp prisoner in order to help Shmuel look for his father, he's forced into a gas chamber along with his friend. In Auschwitz, crossing the fence can literally kill you. Yikes.