With 4 drives and this structure I receive the capacity of 3 drives. The final drive is called the "parity" drive which keeps some kind of copy of the information on it. If one drive fails then I can replace it with a new drive and rebuild the data from parity. This is a long process that requires the data off the parity drive and the other two drives. But you can do this with any disk, from any other three disks.

It's really cool. Sure there are speed benefits but the real kicker is the size of the pool. With current tech I can fairly reasonably get four 18TB drives SSD's have a long way to go before they affordably reach that kind of capacity.