What does Nationalism mean?

Nationalism, sometimes also referred to as "patriotism," is the ideology that a person's primary dedication should be to their nation. A nationalist (should) consider the values and goals of their nation as more important than their own personal interests or loyalty to some other group. In cultural expression, nationalism is portrayed through idealizations of what a particular nation is. For example, the song "America the Beautiful" conjures up images of great expanses of bucolic farmland and untamed wilderness as metaphors for freedom and prosperity. Nationalist art may involve personified forms of particular values or the nation itself. The figure of Marianne has commonly been used in French nationalist art as a personification of liberty and the French spirit. In fact, she is often depicted leading the people during the French Revolution as a sort of mother-goddess figure. 
Nationalism has been the inspiration or direct result of a number of revolutions, including the French, American, and Chinese Communist Revolutions. Unfortunately, nationalism can create a slippery slope into exclusive mentalities. The recent decision for the United Kingdom to leave the European Union was in part due to British nationalism and the belief that immigrants pose a threat to the security of the United Kingdom. This decision has the potential to create great economic, social, and political unrest in years to come.
https://www.bbc.com/news/uk-politics-32810887

https://www.britannica.com/topic/nationalism

https://www.francethisway.com/history/marianneoffrance.php