theremin is motion sensitive instrument. Proximity of your hand in reference to the antenna will varies pitch, in reference to the small disc will vary the volume.
Do a search on leon theremin, lotsa fasicnating story on the inventor and his life and some KGB connection/sabo whatever
Theres lotsa variation of the theremin nowsaday. The smaller one usually dont have the volume control that is proximity controlled. Some comes in kit package which you can build yourself. The standard defacto for serious theremin player, wud be the moog theremin.
Its not easy to use the theremin as it has its own theory of breathing controlled, hearing practise what so ever. To make noise when combined with other efx is easy, but to make a theremin sounding musical and soulful, its beyond my level. I had the LAL soapbox theremin and other than making it do digital moaning, i think iam too stupid to make it sound like how it shud be