The Order determines what sequence Nightly Autoselect will use to move through the preferences looking for books to send out. If preferences are entered as 0, or if multiple preferences are entered with the same number, the system will determine an internal order for those preferences based on when they were entered. Each night that the system looks for books for that patron, it will pick back up right where it left off in the list. This ensures that patrons will receive a mix of subjects or authors, instead of getting mostly books from the first few preferences.
Weight determines how many books it will look for in that preference before moving on to the next one. To keep nightly running smoothly, we usually recommend keeping weight to about 5 or less.
For more information, open the Preferences tab and then press F1 to open the Help page for that tab--it might have answers to questions you haven't even thought to ask!