Barbed wires are a length of steel wire with razor-sharp barbs (thus the name) protruding at intervals throughout it. Infantry crossing through a barbed wire fence would likely risk severe injuries, and light vehicles would certainly have their tires cut open. While it would be possible for an infantryman to slowly and carefully negotiate his way through the wire, or cut the strands of wire to pass, in the heat of battle this would endanger their lives. Therefore, infantrymen and light vehicles usually avoid barbed wire. However, infantry armed with explosives find that, luckily, their weapons could clear a path through the barricade, and heavier vehicles could simply crush it under their tracks.
After the Second World War, difficulty with handling the wires, and the general ineffectiveness of the barriers, leads to the abandonment of barbed wire as a military barricade, though civilian farmers would still use it as an inexpensive way to keep their livestock in check.
Barbed wire was likely meant to be buildable in Tiberian Dawn at one point given the presence of its cameos, left unused, in all iterations of the game. It would have cost $50.