It says that we always say more than we say with words. Although one and the same word is used in all four utterances, different things are meant. The speaker assumes that the exact meaning is known and understood by the hearer, and assumes that the hearer will cooperate by providing the intended meaning, although it is not clearly stated.
Communication only works because we can assume cooperation on both parts and because we can assume shared knowledge on both parts, the hidden meaning thus being understood.
Imagine ordering a new kitchen window: You would be annoyed if the craftsman only inserted the window frame without the window pane, as you would be if a painter painted the window pane instead of the window frame or in addition to the frame. This means that even in everyday conversation we always read “between the lines”.