Hindu nationalism is a belief that people who follow the Hindu religion should be the primary focus of a country's government and culture. This means that they believe that Hinduism should be the most important part of a country's identity and that other religions and cultures should be less important.
Imagine you and your friends like playing with toys more than anything else - this is just like Hindu nationalism, where people who follow Hinduism believe it is the most important thing in a country!
Hindu nationalists often believe that Hinduism is the only religion that really belongs in certain countries, like India, and that other religions are just foreign imports. They also often believe that people who aren't Hindu should not have as many rights as those who are, which is not a kind or fair way to act.
It's important to remember that not everyone who practices Hinduism is a Hindu nationalist - just like how not everyone who likes playing with toys is a toy-only person. There are lots of different people with different beliefs and opinions, even if they share a religion or interest.