This module contians two relays that are electrically isolated from the controlling input. The relays can be used to switch higher voltage and current loads than a microcontroller can traditionally accomplish.
 
Features
Powered from 5V
2 channels
Can be used as Normally Open (NO) or Normally Closed (NC)
Optically isolated inputs
LOW at input enables the Relay
 
The relay has two outputs-normally open and normally closed (NO and NC). When the IN1 or IN2 pin is connected to ground, NO will be open and NC will be closed, and when IN1 or IN2 is not connected to ground the opposite occurs. Connecting a circuit or device between one of these two pins, the common pin on the relay output, and a power source will allow you to toggle power to a circuit or device.