Johann Schmidt's Prosthetic Mask was worn by the Red Skull in order to hide his own hideously disfigured face.
Red Skull's Disguise
When Johann Schmidt, the leader of HYDRA, had learned that Abraham Erskine had developed the Super Soldier Serum which had the potential to transform him into the superior man with incredible abilities, Schmidt decided he must have it in order to ensure that Nazi Germany came out victorious during World War II. When Erskine refused to hand it over, however, Schmidt took it by force and injected himself, only to discover that it was incomplete, resulting in Schmidt's face becoming hideously disfigured while he gained his abilities.
Schmidt's skin was burned off his face, giving him the nickname of the Red Skull. To hide his disfigurement Schmidt made a prosthetic mask that resembled his original face. Although the mask often needed a slight readjustment by pushing at the cheeks, Schmidt wore it constantly. However, when Schmidt was confronted by Captain America at the Austrian HYDRA Weapons Facility, the mask was damaged. As a result, Schmidt ripped the mask off and revealed his true face before throwing the mask into the fire below.