In The Boy in the Striped Pajamas, the fence is a significant symbol because fences are always a means of separation and containment.
Often, too, fences are a means of preventing that which is behind it from entering one's property or interfering with one's privacy. During the Nazi regime, Jews were removed from Germany after several restrictions and taxes were placed upon them. Their property and money were confiscated, and they were removed from Poland and other countries in which the Nazis exerted control. Placed in concentration camps such as Auschwitz--Out-With, as Bruno thinks it is called--they were contained behind barbed-wire fences that are topped with concertina wire, as well.
Huge wooden posts, like telegraph poles, dotted along it, holding it up. At the top of the fence enormous bales of barbed wire were tangled in spirals, and Gretel felt an unexpected pain inside her as she looked at the sharp spikes sticking out all the way around it. (Ch.4)
It is apparent to the children that whoever is behind the fence that is topped with this concertina wire (often called "razor wire") so that no escape is possible are treated as less than human beings. It is also apparent from the distance of this fenced area that there should be no contact made with whomever is contained within this area. Bruno and his sister may assume that those inside this type of fence may be inferior, criminal, not the same, or even dangerous in some way to those on the outside.