This is a classic Zen instruction.
This question was asked by Master Hakuin (1686-1769). Hakuin is credited with aiding in moving Zen into the urban life which was emerging at the time of his life. He writes;
What is the true meditation? It is to make everything: coughing, swallowing, waving the arms, motion, stillness, words, action, the evil and the good, prosperity and shame, gain and loss, right and wrong, into one single koan.
It is said that a young monk given this question to ponder responded by attempting to use his hand to strike a gong and play various musical instruments, all of which were declined as answers by the master. Finally, in a moment of satori, he exclaimed : " I have heard sound without sound!"
The master is conveying the point that sound and other physical phenomenon are just our mind's interpretation of outside stimuli or forces. The second hand is a metaphor for our senses and ego, the sound made is that which is produced when our mind and senses strike with these outside forces and translate them into something we as humans can perceive.
This also demonstrates that without such outside forces, our mind can create an appropriate reaction; the mental "sound" of one hand clapping.