First time using Co2 ?

I am trying to set up the Hydrofarm Co2 system in a garden and can't find out what exactly it means when it tells me to unplug the solenoid valve in step 5 of the directions. Any one who has set one of these up feel free to explain what exactly this means so I can fix this.

The solenoid valve has threads that will fit onto the C02 tank and an A/C cord to plug into the controller. When the valve is energized it opens and when it loses power it closes.