This sentence is a reading and learning method advocated by Confucius. It refers to reading blindly without thinking, because you can't deeply understand the meaning of books, can't use book knowledge reasonably and effectively, and even get confused. And if it's just a dream, if you don't really study and study, you will only build a tower on the sand and get nothing. It warns us that only by combining learning with thinking can we learn practical and useful knowledge, otherwise it will have little effect.