Well a mane res would mean you have say a 40 gallon tub filled with water and your nutes, and it is connected to all of your tubs so you can control all the tubs from one area and control water levels and PPM from that area and not have to mess with every bucket. It is connected by PVC piping or tubes that keep recirculating the water from the buckets or tubs back threw the mane reservoir. It is called a recirculating DWC set up.
The GH Water Farm 8 pack comes with the 8 buckets and a controller bucket and a reservoir. The controller bucket is where you check the PPM, PH and add your nutes and then it has another reservoir that feeds water into the 8 buckets threw the controller when the plants have drank up allot of water.
Adieu