Dream of the Red Chamber: Explaining the World

My name is Liu Yu, a prince stripped of his royal status. I was banished to the frontier. Now I face a problem. The emperor who stripped me of my title as prince, erased my name from the imperial genealogy, and demoted me to a commoner has been trapped on the grasslands during a personal campaign. Now he has issued an edict restoring my title as prince and putting my name back in the imperial genealogy. He has even bestowed on me the title of Prince Zhong'an. When the envoy arrived, I was hauling bricks at Shahu Pass. The key point is that I had only just transmigrated and still didn't identify with this body or anything about it, while the emperor's envoy was explaining the great cause of the realm to me. I really want to know: what is the realm? Does anyone online know? Waiting for an answer, and I'm in a real hurry...