zfs is just a disk file system format (e.g like fat32, exfat, ntfs), z2 for laymen is basically double parity drives (drives are setup that 2 drive worth is used as data integrity, so in the case of a dead disk drive, data isnt lost and can be repaired. z2 offers 2 disks can die before recovery is finished (meaning requires 3 drives to die to actually lose data))