RAID (which stands for 'Redundant Array of Independent Disks') is a technology that uses two or more disk drives cooperating to improve server performance. This can be used in many applications to ensure content is consistently running if one drive so happens to fail.
By default, new servers are setup with RAID 1, which mirrors data on two hard drives. This is not a replacement for making backups, however.
You don't need to do anything to use RAID, just enjoy.
RAID-1 is now included on all of our Shared, Reseller and Dedicated servers.