NOTES: If you want complete optical isolation, connect "Vcc" to Arduino +5 volts but do NOT connect Arduino Ground. Remove the Vcc to JD-Vcc jumper. Connect a separate +5 supply to "JD-Vcc" and board Gnd. This will supply power to the transistor drivers and relay coils.
Having the seperate 5V go to the JD-Vcc means the Opto-receiver and the Relay are totally isolated from the Arduino Vcc. Overkill for sure, but the design provides for that.