The Reichstag is an important government building for the Greater Nazi Reich located in central Berlin. It stands before, and is dwarfed by, the primary seat of power in the form of the much newer Volkshalle, or "People's Hall". It is also used for congresses.
As in real-life, the Reichstag building notoriously caught fire in early 1933, an act the Nazis would blame on communist agitators, which allowed them to suspend many previous rights and laws. The building was not used during World War II in any governmental capacity, though in our timeline it did serve as a focal point for the Battle of Berlin.
In the alternate timeline, the Reichstag was repaired and refurnished following the war, and presumably began to serve the Reich's government once more, in an albeit overshadowed role.