What is an incisional hernia and how can it be treated?

In response to a question on healthshare.com.au

A hernia is where the internal parts of the body - such as the fat or the intestines inside the abdominal cavity (belly) - protrude through a gap or weakness in the muscles over them. The weakness can be natural, for example at the umbilicus (belly button) or in the groin; or it can be acquired as a result of an injury or a surgical incision.