This unusual performance is an old Chinese tune called Sai Ma (Horse Race) composed by Huang Haihuai. Lang Lang is playing the piano and his father is playing an Chinese instrument called an erhu. The erhu is a two-stringed bowed musical instrument, sometimes known as the Chinese violin or a Chinese two-stringed fiddle.
The erhu is used as a solo instrument as well as in small ensembles and large orchestras. In China it is used in both traditional and contemporary music arrangements (including pop, rock and jazz).