This quote can be related to nationalism in that it advocates that all of the people of a country should be united. This is more of a fascist idea, but it can also be connected to nationalism.
Nationalism can be defined as the idea that one's own country and people are superior to others and that one's loyalty should be to the country and its (in Hitler's type of nationalism) dominant ethnic group. Implicit in this definition is the idea that the nation and the people should have something very much in common that binds them together. They should be united. Hitler is saying that the essence of leadership is keeping this unity and focus intact.