RAID1 Mirrors the data across multiple disks (Minimum of 2). By spreading the data across more than one disk should you suffer a fault (Drive failure) your drive controller would simply send you an alert that a drive has failed and that you should replace the failed drive at your earliest convenience. There are speed benefits compared to a single drive however you loose the capacity of the mirrored drive.
For example – two 10tb hard drives in RAID1 will only give you the capacity of 10tb as the second drive would be used for backup of the first.
Simply put – Mirroring gives you a mirror of the drive at the expense of capacity.
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