Pre-authorization is a process used by hotels all around the world to guarantee guests reservations.
Pre-authorization is only used for reservations which use the Pay at Hotel payment option. The process does not charge your card, and only reserves 1-night cost in order to validate that your credit card information is correct. The refund is issued within a very short time after the original charge (less than 1 minute), and before the funds actually move from your bank account.
The money should be released and reflected in your bank statement straight away. If you still don't see the funds back to your account, it is advised that you reach out to your bank, as the delay can be caused by their internal procedures.