The PC817 is an optocoupler or opto-isolator, which is an electronic component that uses light to isolate two circuits electrically while allowing data or signals to pass between them. The PC817 optocoupler consists of an infrared LED and a phototransistor or photodiode, housed in a single package. When an electrical signal is applied to the LED, it emits light, which is then detected by the phototransistor or photodiode. The resulting electrical signal from the phototransistor or photodiode is then used to control the output circuitry.