Adolf Hitler was an Austrian tyrant in the Real World and many other continua. His name is synonymous with evil in most RL cultures that know of him, on account of the millions of people he had murdered for eugenic purposes. He was the leader of the German Nazi party.
When Hitler is featured in a series or fanfic that involves time travel, there will often be an attempt to prevent his rise to power or kill him to prevent the Holocaust. These often fail, or somehow manage to make things worse.