von Willebrand disease is the most common hereditary coagulation disorder. It arises from either qualitative or quantitative deficiency of von Willebrand factor, which is a protein that is required for platelet adhesion.
When an injury occurs the sub-endothelial collagen becomes exposed. This leads to release of Von Willerbrand factors, which act as a linker molecule for platelet to bind to the collagen using Gp1b, which is located on the surface of platelets. Therefor, if there is no Von Willerbrand factors, that initial step for platelet adhesion cannot occur and the patient will not be able to stop bleeding and will present with Mucosal and Skin bleeding.