It means that a disk goes bad and you lose the data. Typically there is some form of protection. I use standard raid 10 which is a bit dated but modern approaches like erasure coding are getting more common. Even if it’s JBOD, you should have a copy of the data in case a drive dies. That’s the value of like raid 5 since it gives you most of the drive space and tolerates a drive failure. RAID is available in software but I’m still using older LSI hardware controllers. A RAID1 mirror would basically be similar to just copying files from one drive to another manually. You get half the storage space but don’t panic when a drive dies. The thing is that drives do die. They are viewed as consumables and thus the question is always WHEN not IF they will die.