Warning - mild early episode spoilers.
This question keeps coming up on the thread so instead of just reposting it over and over, I am just going to leave it here:
The pearl tribute is a pearl tax that is used to fund the emperor's coffers. It should be in exchange for a concession to harvest and trade the pearls bc since ancient China, pearls were considered treasures of the emperor. This drama is set in Tang Dynasty where Hepu was the center of the pearl trade. There is apparently quite a well known a professor of history on the production's research team.
https://www.meetpearls.com/blogs/news/meet-the-history-of-pearls-in-china
At a high level, there are two opposing factions in this story: Prince Ying and Prince Tan. Most likely they are fighting for the throne/crown prince position.
The Cui family own the pearl farm and they have to send tribute pearls, which are a pearl tax to the emperor. But they have been under-reporting tribute pearls which are very valuable and probably selling them to fund Prince Tan's military buildup. Both Envoy Xu and the Cui family are in Prince Tan's faction but now that they are about to be exposed for illegally selling tribute pearls, they are kind of starting to turn on each other.
Meanwhile the Cui family murdered Yan Zijing's family and took over the pearl farm from them so he is out for revenge. He is inciting Zhang Jinran who is Prince Ying's sworn brother to investigate the Cui family and take them down. At the moment, its not clear that Zijing is on Prince Ying's side. He is just out to get the Cuis. But they are so powerful that it is going to take more than some pearl tax evasion to take them down. They are rich enough to buy back the tribute pearls to make good the tax. That is what they are trying to do but Zijing is trying to make it super expensive for them by cornering the pearl market.
Throughout Chinese history, there were quite a few noble clans that exerted significant influence over the court for centuries, outlasting different dynasties because they are so critical to the economy that each emperor always needs their influence. The Cui family, Yan family, Wang clan of Langya etc are among them and you will see them pop up in many period dramas.
