The EKM-Switch120 is a normally open switch (no electricity can pass through the 120V portion of the unit).  When you apply a low DC voltage to the control ports (ports 1 and 2) the switch closes and the 120 volts becomes energized.

On our website here: EKM-Switch120

Port 3 is ground from your 120 volt AC system — you do not need to use this.

Port 1 is low voltage Positive.  Port 2 is low voltage Negative (or ground)

To close the switch you will apply from 3V DC to 12V DC between port 1 and port 2.  To open the switch you will stop applying 12VDC between port 1 and port 2.