Parity initialization occurs when parity is calculated for every stripe on a RAID 5 volume. Parity is a mathematical method for re-creating data that was lost from a single drive, which increases fault tolerance.
On a normal shutdown, all parity operations are completed before the system is allowed to power down. When a power loss or system crash occurs, there may not be enough time for all parity operations to be completed. This results in a dirty shutdown because parity may be inconsistent with the data it is supposed to be protecting.
On the next reboot after a dirty shutdown, parity must be completely initialized in order to guarantee that it is valid for each stripe.