Like RAID 0, a RAID 5 array breaks data into sections that are stored on two or more drives, resulting in increased speed and capacity. But it devotes one additional drive to parity, saving information on the other drives' information.
If one of the non-parity drive dies, the parity drive can look at what's on the working drive (or drives), and calculate what would have been on the dead one. The RAID and the computer will continue to work, although the extra calculations will slow things down.
Read the original forum discussion.