What do you mean by immigrants guarding the border?

It refers to the policy of moving mainland residents to sparsely populated and desolate border areas, which can develop agriculture and economy in border areas and strengthen national defense forces. This rule has been practiced in many dynasties in history. For example, during the period of Emperor Wu of the Han Dynasty, hundreds of thousands of people moved to northern counties in order to strengthen the northwest frontier defense, and the border economy and culture developed rapidly.