In case you need a refresher on Captain America: The First Avenger, Captain America and Red Skull had their last battle aboard the plane carrying HYDRA’s weapons of mass destruction, and during the fight, the container carrying the Tesseract was damaged.
This time, however, the scene is illustrated to make it look like Red Skull was transported somewhere else in the universe.
Panicking, Red Skull grabbed the Tesseract, which resulted in the flesh on his hand burning away, followed by a portal opening up that he then disappeared through.
While it seemed as though Red Skull was simply disintegrated due to making direct contact with the Tesseract, Marvel’s Avengers: Infinity War Prelude #2 (via Screen Rant) suggests that wasn’t the case.