High mountain, Flowing Water-高山流水（gao shan liu shui)

初志在乎高山，言仁者乐山之意。后志在乎流水，言智者乐水之意。

This song is about a friendship between two guys (Yu Boya and Zhong ziqi) in the Spring and Autumn period. They had exactly same music taste and understanding. One day, Boya played a tune in which he wanted to express the loftiness of mountains. After his performance, Ziqi said: "It's for the mountains." Then Boya performed another one, Ziqi said: "It's for flowing water."

After Ziqi died. Boya broke his lute and vowed never to play the lute again because he thought there would nobody in this world could understand his music.

So Gao Shan Liu Shui is also a metaphor to describe close friends.