This is an SSR-60VA Solid State Relay Module. A solid-state relay (SSR) is an electronic switch, which, unlike an electromechanical relay, contains no moving parts. The types of SSR are photo-coupled SSR, transformer-coupled SSR, and hybrid SSR. A photo-coupled SSR is controlled by a low voltage signal which is isolated optically from the load.
The control signal in a photo-coupled SSR typically energizes an LED which activates a photo-sensitive diode. The diode turns on a back-to-back thyristor, silicon-controlled rectifier, or MOSFET transistor to switch the load.
In this Particular SSR, the power output is controlled by the trigger angle of the triac with the variable resistor of 470-560KΩ. Each one of these relays is equipped with four screw terminals (for use with ring or fork connectors) and a plastic cover that slides over the top of the relay to protect the terminals.