When you click Complete this reservation on the Reserve screen, you see the Booking confirmation screen.
You also receive an email informing you that the booking has been successful.
Choosing Reserve, Select and reserve or Complete this reservation does not necessarily mean that a reservation is guaranteed for the service immediately. Many company settings include a process for approval (for example, by a line manager or supervisor), in which case the service must be approved by the date indicated, otherwise the reservation will be cancelled. (This does not apply in the case of bookings with certain low-cost carriers where tickets are issued immediately upon reservation.)