RAID (*R*edundant *A*rray of *I*ndividual *D*isks or *R*edundant *A*rray of *I*nexpensive *D*isks) is a standardized system for using 2 or more disks to appear as one volume, working together. In the case of RAID 1, the RAID Controller synchronizes all the read and write operations between 2 disks. You only get use of 1/2 the storage using RAID 1.
Because the writes are all synchronized, RAID 1 is not a disaster recovery method of backup; you should still take proper backups to ensure against data loss. If a disk fails, however, the operating system can still access all the data off the other drive until it fails as well.