Without being too technical, protected drives (RAID) means there are two (2) physical hard drives that “mirror” each other. If one fails, the other drive has “a mirrored copy” on it. The goal is to take a worst-case situation (hard drive fails) from the lengthy process of order new parts, reload everything, download from the backup system meaning multiple days to move a cable to another drive, and restarting the computer (hours or less). RAID does not replace online backup because it is still possible to lose both drives in a fire, flood, theft, etc.
Geeks will say RAID eliminates “a single point of failure” – if I build porch and use (2) 2 x 4 nailed together and one cracks – porch may not fall – compared using on one, the whole thing probably will come down.