It is basically as your friend said.
A raid (Redundant Array of Inexpensive Disks) 0 array allows you to put two (or more) identical HDD in the PC, and set up correctly, the two drives will both contain identical info. So if one drive dies, the info is all on the other.
It does not stop the system crashing, a virus trashing it, or an eletrical surge blowing it all up, but it does protect against one off disk failure.
But you still need to backup, incase of the virus/crash/surge.