You cancel confirmed bookings and reject bookings that are not yet confirmed.
Further explanation:
To cancel a booking will be for a booking already confirmed.
To reject a booking will be for a booking still in progress. Rejected bookings will show as cancelled bookings after instead of confirmed.
To delete a booking you will require user permissions. Deleted bookings will be invisible on the system for the users that are missing this user permission.
For further information on booking status please see the article Enquiries Page - Booking Status Explained.