RAID is a way of having your information on your computer stored on several disks. If something happens to one disk that you do not lose your information). However if something happens to your computer RAID does not protect you.
RAID is only protects you if a disk fails. Backup protects you if anything happens to the computer including complete destruction of the computer. It is important to keep your backups at a separate physical location form the computer.
RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. RAID has been around since the 1980s and at that time what they saw as large inexpensive disks we would laugh about now. The sizes were small in storage and in terms of today where a terabye is less than $100 they were absurdly high (hundreds per megabye in 1980s), but cheap then.
There are several kinds of RAID. With RAID 1 or mirroring there are two identical disks in the PC and everything is written to both so they both have all the information. With RAID 3 and 5 the stuff is written once but it is scattered over 3 or more disks (for speed) and a key to the data is written on disk that the data is not written on. If a disk fails we can regenerate the data from the data if the disk with the key failed or if one of the others failed, the key gives us a shortcut combined with the data known to recreate the data. With RAID, if two or more disks fail, RAID will not allow us to recover the information and we will need a backup.
There is a lot more on RAID and other levels, but I am trying to just give you a brief introduction to what RAID is.