Thinking that the swan is coming means that a swan is about to fly over.
This verse comes from the study of chess in "Mencius Gaozi". This article mainly talks about Yi Qiu teaching two people to play chess. One of them studied seriously, but the other only focused on playing chess. Think of a swan flying over and shooting it down with a bow and arrow. The first person learns well and the latter learns poorly. It's not because the latter person is smarter than the former person, but because his mind is not on playing chess.
This sentence now refers to a person who only has lofty aspirations but does not take practical actions. We should not only have great ambitions, but also have the determination to work hard and down-to-earth to realize our ambitions.