RAID combines several physical disks into one logical disk. Most RAID levels (e.g. RAID 1, 5, 6, 10) spread data so that the server keeps working if one disk — or, depending on the level, two — fails.
Important: RAID is not a backup. If a file is deleted by mistake or encrypted by ransomware, the change is immediately written to every disk. RAID protects against hardware failure; backup protects against data loss.
Example
A server with two disks in RAID 1: one disk fails, nobody notices, the disk is replaced and the data rebuilds automatically.