In The Burning Forest, a collection of articles and essays by Simon Leys, the chapter with the above title explains the metaphor.

“Twenty-three hundred years ago, Zhuang Zi, in  giving advice to a king, made him observe that when a small boat drifts in the way of a huge barge, the crew of the barge will immediately shout abuse at the stray craft… if they discover that the little boat is empty, they will simply shut up and quietly steer clear of it. He concluded that a leader… should first and foremost learn how to  become an empty boat.

History provides few examples of statesmen who were as successful as Zhou Enlai in mastering this subtle discipline.”