The Marvel Cinematic Universe is a franchise consisting of film and television adaptations of Marvel Comics publications produced by Marvel Studios. The franchise is notable for being the first superhero film franchise to have characters and plot points from different titles crossover.
The Red Skull is the original head of Hydra and is Captain America's arch-nemesis, portrayed in the MCU by Hugo Weaving. He used the same serum that Captain America did, but an earlier version, causing the side effect of his face skin burning off and his skull turning red.