What are optocouplers, voltage circuits isolation, optocoupler as a switch

 DIP6 SMT 1 Channel 400 V 4170 Vrms Zero-Cross Triac Optoisolator

https://www.futureelectronics.com/p/semiconductors--optoelectronics--isolation-components-optocouplers/moc3043sr2m-onsemi-4354221