RAID 1 (Redundant Array of Independent Disks) is a real-time backup solution where two (most common) or more hard disk drives (HDD) store exact copies of the same data at all times. This setup is also commonly referred to as the mirroring of data. If one HDD fails a complete copy of all data is available on the other.
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If a personal computers operating system (OS) is stored as RAID 1, a PC can continue functioning normally until the faulty HDD is replaced. After a disk is replaced the RAID controller rebuilds the mirroring of data while the system continues to operate.
RAID 1 provides data redundancy at the cost of storage capacity with total capacity being the capacity of one of the HDD (equal capacity) with the OS seeing the RAID array as a single disk. There is also some loss of performance due to the extra HDD read/write activity to comply with RAID 1. Hardware implemented (eg. by computer mother board) RAID 1 has less performance loss then software implemented RAID 1.
There are other RAID levels (configurations) which have different balances between data redundancy, capacity and speed performance with RAID 0 and 5 being the most common.
