Let's wait for a Chinese member here to explain it in a humorous and knowledgeable way in good Chinese. The English footnote which I can provide here is this: It's jokingly imitating Emperor Kangxi's habit of marking officials' reports to him with "朕知道了" - 朕 is the first person pronoun reserved for the Emperor. It's netizens' way of playfully responding to friends with "Yours truly the Imperial Majesty has noted your message." "你退下吧" is in the same vein. It was the Emperor's way of ordering someone to leave: "You may now be excused." Some people make political jokes out of this. Others play Emperor and Concubine with their girlfriends. It's the internet and the world of instant messaging.

朕：皇帝的自称 the Chinese emperor call himself "朕" after the emperor finishing listening to the reports of the bureaucrats or someone else(Queen/concubines ……), he would say the sentence to express that he got the point of the reports and the guy(bureaucrats or someone else) should stop speaking and leave now. Hope this will be helpful! :)

This sentence is also used by some Chinese people in daily conversations with close friends and sometimes with family members. The tone of emperor in this sentence makes our conversations suddenly funny and makes people feel the connection between ancient Chinese and our daily life. Also, because many Chinese people now watch TV series about ancient Chinese royal family where sentences like this are widely used, this kind of sentences(with the tone of the emperor or queen……) became popular in our daily life.

The emperors/皇帝 called themselves '朕'(zhen4) from Qin Dynasty(2200 years ago) to Qing Dynasty(100 years ago). Before Qin Dynasty, everyone could call himself '朕', it's like ‘我’/'余'/'吾'/'俺'. Since the emperor used '朕', the normal persons were not allowed to use '朕'. Now It's a internet slang or humourous way to call oneself. '朕知道了 你退下吧' may come from a TV series whose action took place in the ancient imperial palace.

“朕” means “I”， "退下" means "go out"。 It was used in the qing dynasty, about 300 hundreds yeas ago. Only the King can call himself "朕". That ususlly appears in a costumedrama, we seldom use it in our daily life.