This quote is from Doctor Zhivago. It takes place while Yuri is in the camp and the war is raging between the whites and the blacks (the revolution). Yuri is grieving because he wants to be with his family and Lara. He is worried about what is happening to them while he is away. This quote is both a simile and a metaphor, comparing an old Russian folk song to water held back by a dam and to "an insane attempt to stop time."
I believe it refers to the spirit of the Russian people. First of all, because the song is a Russian folk song. Folk songs often express very deep thoughts about life, love, emotions, history. Often the song appears to be a mere ditty, but the meaning is much deeper, like water that appears to be calm on the surface, but is really quite deep. In a folk song, refrains are often repeated and repeated. These refrains focus on what the song is about, what is important in the song. If one focuses on the refrain and thinks about it, one can discern the meaning. When this happens, it is as if the floodgates open. It is an ephiphany. One figures out what the song is about, and it is profound. One gets a glimpse into the sorrow and spirit that was behind its writing. A folk song may be trying to teach a lesson, it may be trying to get peoples' attention and say: LOOK! Pay attention! Watch out! Don't be fooled.
Pasternak is expressing his feeling of hopelessness, however, that a song, just like the Russian people, cannot stop time with their words. To do so is insane. It is his comment on the grief and sorrow the common people experienced not only under the Tsar, but afterwards, under the leaders of the bloody Russian Revolution. If you understand the history of Russia, you will see that the Russian people have suffered for centuries, yet they are resilient.