As far as I know, there is no technical difference between "ultranationalism" and just plain nationalism.  The former is just a more extreme version of the latter.

Nationalism is the idea that there is something unique and superior about your ethnic group or your nationality.  As this gets to be more extreme, it starts to include a violent hatred of another group or groups.  So Nazism would be an example of this.

It usually arises in a society when the society has fallen on hard times.  People look for scapegoats and often pick a group of "foreign" people to blame for their problems.

This is important because it can lead to attempts at ethnic cleansing or even genocide.