Parity is a common way of detecting errors in a storage system. This is a way of adding checksums into data that can detect errors during data transmission. The way parity works is by adding a single bit of data to the end of a data block to ensure that the number of bits in the block is even or odd. This lets the receiving device know that a message with no errors contains a certain number of bits. If the number is different then the source device must resend the data.
In certain RAID configurations, such as RAID 5, there is vertical and horizontal parity. This means that a disk or set of disks contain parity information that allows them to rebuild data in the event of a drive failure. In a RAID configuration, data always will have correct parity because it has been run through multiple error checking algorithms.
To detect this error and begin a rebuild the array will begin identifying what was on the failed drive by adding up the bits on remaining devices. If the remaining data ends up to an odd number then the information on the failed drive must have been a 1, while if the data adds up to an even number it must have been a 0. The RAID array then takes that information and begins to rebuild the data from the failed drive.