The book uses the line to illustrate the importance of taming, a metaphor for forming a meaningful connection with another being. The fox explains that only through taming can you truly see the world and appreciate its beauty.
The quote is often misremembered as "What does the fox say?" This phrase is actually from the popular children's song "The Fox (What Does the Fox Say?)" by Ylvis.
So, while the fox's famous line is "Good morning," the question "What does the fox say?" is a catchy and playful song that has nothing to do with the book.