It means: in this spring March full of catkins, go on a long trip to Yangzhou. The lonely sail shadow of my friend faded away and disappeared at the end of the blue sky, only seeing a line of the Yangtze River, heading for the distant sky.
The original sentence comes from Li Bai's Farewell to Meng Haoran on the way to Yangzhou by the Yellow Crane Tower in the Tang Dynasty. The first sentence points out the place of farewell, the second sentence writes the time and destination of farewell, and the third and fourth sentences write the scene of farewell. The whole poem contains parting thoughts. Against the background of colorful and mottled fireworks and the endless Yangtze River, a farewell picture of a poet with broad artistic conception, endless emotions, bright colors and charming charm was drawn.