In simple terms, a mirrored volume (RAID-1) writes the same data on two disks or partitions of separate disks. That is, any changes to data (or new data) made to the first disk of the mirror set are also made to its mirror disk. Mirrored volumes require at least two disks or two free partitions on separate disks. The free partitions or disks need to be of the same size, otherwise, the mirrored volume size would be equal to the smallest partition or disk.
The main advantage of mirrored volumes is disk drive redundancy. It provides availability of data in case one disk drive fails while it can mirror a system disk containing the operating system. If a disk fails, the mirror set continues to operate on the remaining disk. The recovery operation of the mirror set depends on which disk fails if the set contains the system disk. In general you would break/remove the mirror set, replace the faulty disk and re-create the set.