Barbed Wire is a defense kit.
There are two versions of barbed wire (one that is slightly taller than the other) but functionally they do the same thing.
After placing it down it acts as a trap for the zeke to run into. Sometimes they will fall into the wire, wriggle around for a bit, and then die because they cannot escape. As zombies pile up into the wire, the durability decreases and zombies will be able to use the corpses already in the wire as a bridge to climb over it completely.
It is important to note that Bulls completely ignore Barbed Wire when jogging at you, as they always climb over it no matter what, and when charging you, as they will stampede their way through.