RAID stands for Redundant Array of Independent Disks, and it’s great for backup purposes. The word ‘redundant’ comes from the fact that the same information is written to more than one hard disk, so if an active disk fails then one of the redundant ones that’s been sitting around doing nothing will come to the rescue. If your website data is held in lots of different places, then it means you’ve always got a backup if one disk fails. Note that not all RAID types offer this kind of protection, so it’s always best to check.
Please note that technologies described on Wiki pages are not necessary the part of Plesk control panel or its extensions.