Depending on who you talk to RAID is an acronym for "Redundant Array of Independant Discs" or, "Redundant Array of Inexpensive Discs". Either one will work in a pinch. There are basically two types of RAID. One involves mirroring and the other involves striping. More comprehensive types involve combinations of both with or without various sorts of caching. RAID 0 is strictly striping. In layman's terms it widens the information highway you burn onto your disk so that more data can be written faster. This allows you to put two or more discs/drives together to build a much larger disc/drive in the sense that it can have a greater capacity. Drawback: Your disc/drive fails, you lose all your data on that disc/drive. RAID 1 involves mirroring. It writes an exact duplicate of one drive to the other drive. The process takes time but it certainly can protect your data via redundancy. RAID 5 gives you a bit both and you must use a minimum of three drives. RAID 10 will give you a lot of both, that is to say both speed and redundancy and you have to use a minimum of 4 drives as well. Drawback: You need a number of discs, preferably all the same size, make, and model.
Essentially your external drive is asking you to select the discs you want to use to mirror your data it seems. My Seagate does the same thing. There should be a setting in there for opting out of that. I opted out of using this as I already have my mirrors running on an isolated Broadcom RAID array. You should not have to create a RAID array to simply back up your data although it is not an entirely bad idea.
I realize this is an over simplified explanation of RAID but it will give you the basic idea. I'm guessing that you may be using an external hard drive for back up and it is offering you the opportunity to mirror your data (or networked data) on the disc drive in your NAS. Although technically, RAID is not backup it can act as backup under certain situations involving mirroring. In such a case I still recommend backing up your back up on and independent drive. When it comes to backup more is better. Ideally you should also have a cold storage backup if you seriously value your data but that is an entirely different subject. I hope this helps.
