Localism in Hong Kong is like when you really love your home and your family and you want to protect them. In Hong Kong, some people have started to feel like they want to protect their city and their way of life from outside influences. They want Hong Kong to stay unique and special.
Think of it like a little village. The people who live there know each other and have their own way of doing things. They might have certain traditions or customs that are different from other villages nearby. But sometimes big cities or companies come and try to change things. They want to make everything the same everywhere, and some people in the little village don't like that.
In Hong Kong, people are afraid of losing their freedoms and their culture. They don't want to be just like mainland China, which is a neighboring big country. They want to keep their own laws and way of life. So some people have started to speak up and fight for Hong Kong to have more control over its own affairs. They want to make sure that Hong Kong stays special and unique, just like a little village with its own customs and traditions.