A RAID drive is an external hard drive that contains two or more individual hard disks. (Just in case you were wondering, RAID stands for Redundant Array of Independent Disks.)
This isn't just to give you even more storage space: it's even smarter than that. With two hard disks inside and a clever controller processor, when you back up to a RAID drive, it automatically copies your data to both hard disks. Should one of the disks inside ever fail, you still have all your files safely saved on the other disk.