National assimilation means a group of people who live together in a country learn to be more like each other so they all have similar values, beliefs, and culture. Imagine you are in a big classroom with lots of children who come from different countries and speak different languages. If everyone in the class starts learning the same language, sharing the same holidays, and learning the same values like kindness and respect, then they are all becoming more alike. This is what national assimilation means for a country, where everyone tries to get along and be more similar to each other.