"The instrument's controlling section usually consists of two metal antennas which sense the relative position of the thereminist's hands and control oscillators for frequency with one hand, and amplitude (volume) with the other. The electric signals from the theremin are amplified and sent to a loudspeaker."
Basically, you move your hands up and down above it to control the sound. One hand for volume, one for pitch. Just look at the videos:
Invented by a Russian physicist, Leon Theremin, it was patented in the U.S. in 1928. It sounds so creepy and awesome. I learned about it because it is used to play the very unsettling and unique theme song for the long-running BBC show Midsomer Murders. You can watch the thereminist from that show play below.
