MOC-3021 is a triac driver zero-crossing optocoupler. Optocoupler is made of light-emitting and light-sensing devices with no internal connection except light bean. The optoisolators allow two circuits to exchange signals without making any electrical connection. 

This device works on the principle that light is proportional to signal and signal is transferred to a phototransistor. Opto-isolators or Opto-couplers are made up of a light-emitting device, and a light-sensitive device, all wrapped up in one package, but with no electrical connection between the two, just a beam of light. The light emitter is nearly always an LED.

The light-sensitive device may be a photodiode, phototransistor, or more esoteric devices such as thyristors, TRIACs etc.

Specification: 

Reverse Voltage	3V
Continuous Forward Current	60mA
Total Power Dissipation	300mW
VER Off–State Output Terminal Voltage	400 Volts
Peak Repetitive Surge Current	1A
Number of pins	6
Package	DIP-6
Country of Origin	China