The fire at the German Reichstag (parliament) in 1933 was instrumental in helping Hitler to gain power. On February 27, a Dutch immigrant and ardent Socialist, Marinus Van Der Lubbe, set fire to the Reichstag in the hope that it might inspire the German working classes to revolt. Hitler used this idea to his advantage: he claimed that the country was under threat from the Communists and convinced President von Hindenburg to issue an emergency decree. This decree, known as the Decree for the Protection of the People and the State, took away popular constitutional rights, including that of free speech and freedom of the press. It also allowed the arrest of any potential enemies of the state, a right which Hitler exploited to remove the political opponents of the Nazi Party.
More generally, Hitler also exploited the situation to boost his credibility and prestige among the German people. He portrayed himself as a national hero whose actions in the aftermath of the fire had protected the country from the Communists.
For more information, please see the reference links provided.