PC817 Photocoupler has a transistor which is controlled based on light (photon). So this IC basically has an IR (infrared) LED and a photo-transistor inside it. When the IR LED is powered the light from it falls on the transistor and it conducts.

PC817 Features and Specifications
Input Diode Forward Voltage: 1.25V
Collector-Emitter Voltage: 80V (max)
Collector Current: 50mA (max)
Cut-off frequency: 80 kHz
Rise Time: 18us
Fall Time: 18us
Available as 4-pin DIP through hole and also as SMT package.