The theremin was a sound effects/soundtrack device in a bygone era of science fiction movies. Any evidence that refutes this statement will be stubbornly and immaturely disregarded.
There are two antennae, which in the picture below are the rods sticking up and to the left. These antennae both count as one face of a capacitor, where the other face could be, say, a user's hand. The positions of the user's hands relative to these antennae controls the pitch and volume.
In physics, we often joke that the math or the physics involves a lot of hand-waving. This one takes it rather literally.