So, let's say that you really love your family, your house, your school, your town, your state or your country so much that you want everyone else to love it too. That's what being a nationalist means in a way. Like when you wave your country's flag and sing its anthem, you feel proud of where you come from and you want others to feel this way too.
Being a nationalist means that you think your country should be the most important thing, more important than what other countries think or want. This is why sometimes people who are nationalists might not like people from other countries or cultures, and they may even think they are better than them.
Nationalism can be a good thing because it makes you love and care about your country, and want to do things to make it better. However, it can also be a bad thing if it makes you hate or harm other people who are different from you. It's important to remember that every person and every country is unique and special in its own way, and everyone should be treated with respect and kindness regardless of where they come from.