Assimilation is when one group of people tries to make another group of people be more like them, even if the second group has their own language, culture, and ways of doing things that are different.
When France was a big colonial power a long time ago, they wanted the people in the countries they colonized, like Vietnam, Algeria, and Senegal, to be more like French people. They made laws that said that people had to learn French language and culture, and they pushed aside the local languages and cultures.
They thought that if everyone was French, it would be easier to govern the colonies and that everyone would be more alike. But this isn't always a good thing because it can hurt people's feelings and erase parts of their identity. It's important to respect and value the diversity in the world.