Nationalism comes from the word "Nation". a Nation is a group of people of (usually, but not always) the same race, history, culture, language and geographical territory.
Nationalism is the belief that every nation has the right to rule independantly the land that they lived on for the whole history.
Nationalism was the reason for many wars for independance in many countries.
America is NOT a nation, because it is an immigrant country, and each immigrant has his own race, history and culture (that they often try to keep).