Essentially are holes(defects) in the muscular fascial layer of the abdominal wall through which fat and intestines can protrude causing pain and eventually incarceration(getting stuck) and strangulation(so tight that blood cannot get into the intestines causing necrosis (gangrene).
Umbilical hernias are very common resulting in defects left from the umbilical vein not completely fibrosing after delivery. I generally repair these under local anesthesia with intravenous sedation. If the defect is smaller than 2 cm it is closed with sutures without mesh and if it is greater than 2 cm mesh should be used to prevent excessive tension and hernia recurrence. The intra-abdominal side of the mesh has a protective layer to prevent the intestines from adhering to it.