RAID stands for 'Redundant Array of Independent Disks'. It is a system for providing real-time resilience against hard disk failures where the contents of the server's hard disk are mirrored onto many disks.
All our Miniservers VMs™ and twin-disk dedicated servers come with RAID(1) mirroring as standard. For servers with more than two disks we use RAID(5) or RAID(6).