RAID is a method of using multiple hard drives to either expand the total storage while treating them as one device, or (sometimes and) providing redundancy in the case of hard drive failure.
The simplest version is a pure mirror - one hard drive holds exactly the same information as another. Other versions chop up the HDD storage and then combine them into one big virtual drive. Sometimes these are set up in such a way that the loss of one drive doesn't result in catastrophic data loss - the data can be reassembled when the drive is replaced, but these methods are not really meant for redundancy.
Honestly, the simplest method is probably just an external backup. They typically plug into a USB port and have configuration software that you use to identify which area(s) of your hard drive(s) you want to back up onto them, and then set the frequency of the backup. They'll pull the files on a regular basis and you can restore directly off of them if something happens.