RAID stands for Redundant Array of Independent Disks. In essence it is a series of hard disks that use redundant disks to ensure that if one breaks, no data is lost and the system can continue to function without downtime.
By using multiple disks often the read and write performance is improved as well. There are many different configurations from Level 0 to Level 7.
The simplest configuration is two disks that mirror all the information, more advanced configurations distribute data across all disks, with the possibility to restore/recalculate the data stored on a broken disk

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