Duke of Zhou is a book that records ancient people's dream interpretation. In ancient times, people thought that dreams were omens and information given by heaven, which could help people understand their own situation, predict the future and handle affairs. Therefore, after having a strange dream, many people will look up the explanation in Duke Zhou to understand the meaning behind the dream.
Because we have all kinds of dreams when we sleep, we are called "Duke of Zhou". It means that people inquire about their dreams in their sleep, just like asking the Duke of Zhou for instructions, in order to know the meaning and omen.