Japanese nationalism is when people in Japan feel very proud and protective of their country. It's like when you love your favorite toy or your favorite food and you wouldn't let anyone take it away from you.
Some people think Japan is special because of its history, culture, and traditions. They want to keep their country the way it is and not let other countries influence it too much. They also sometimes believe that Japanese people are better than people from other countries.
At certain times in Japan's history, such as during World War II, this nationalism got really extreme and caused a lot of problems. But today, most people in Japan just feel a normal level of love and pride for their country.