This sentence comes from irony. When a general speaker speaks, he can only understand the superficial meaning, which is called the primary audience; People who can understand the implied meaning of words are called secondary audiences, which are generally jokes played by the speaker and the secondary audience on the primary audience. This sentence was originally derived from here.
Give a simple example: plane geometry contains a rectangle, and a rectangle contains a square. We can say that in plane geometry, a square is contained in a rectangle.
In fact, there are many modifiers in this sentence. Simply put, the superficial meaning is hidden in the superficial meaning.