What is underground cable?

I think you are referring to the practice of burying cables underground, rather than stringing the wires on poles above the ground. The reason some people (and some countries) prefer to place the cables underground is that when there is a bad storm, like a hurricane, people won't lose power because the cable won't be blown down by the wind. Burying cable underground is expensive, however, and as a result, many cities continue to have telephone wires and cable above ground, attached to poles.