An electronic device that converts light into an electrical signal. When modulated by visible light it produces electrical impulses that can be amplified to drive audio speakers.1 Some cells produce electricity when exposed to light; others react to light by offering an electrical resistance.
https://www.nfsa.gov.au/sites/default/files/uploads/2010/09/28/Schematic_of_optical_sound_reproduction.gif
References
1Kodak: A Concise Glossary for the Non-Technician