[Explanation]: A metaphor for outstanding writing talent.
[Source]: Wang Renyu of the Five Dynasties, "The Legacy of Kaiyuan Tianbao· Dreaming of Flowers on the Pen": "When Li Taibai was young, he dreamed of flowers growing on the pen he used, and he became famous all over the world."
[Grammar]: formal; used as a predicate; with a complimentary meaning