To expand on that, RAID is used for performance purposes. RAID is NOT backup. You should still backup your files even if you are running RAID setup.
By storing data across multiple drives, it allows for multiple access points and improves access time. In general, outside of RAID 0, you will end up losing a full drive's worth of space in a RAID setup for the RAID array. Please know that all drives in RAID should be the same capacity. If not, it will run them all with the LOWEST storage capacity among the array.