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 lengthy process of order new parts, reload everything, download from backup system meaning multiple days to moving a cable to other drive and restarting 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, whole thing probably will come down.