Explain operation of solid-state relay
The relay is constructed from an infrared LED which is optically coupled to the paralleled light-sensitive gates of an N channel and a P channel enhancement mode MOSFET with their channels wired in series. 

CORRECTION:

The relay is constructed from an infrared LED which is optically coupled to the paralleled light-sensitive gates of two N channel enhancement mode MOSFETs with their channels wired source-to source in series opposition. 

When the LED illuminates the MOSFETS it enhances their channels, causing the resistance of the relay's output to fall to a low value, and when the LED is extinguished the resistance across the output terminals will rise to a high value.

Consequently, with enough current flowing through the LED the relay will be turned ON, and with no current through the LED the relay will be turned off.

Unlike a common magnetically operated relay, the actuating current is unipolar, but like a common magnetically operated relay, the current through the MOSFET "contacts" may be either direct or alternating.