Welcome to Insiders,mdjst5! Rollover nights are any qualifying nights over and above the annual minimum requirements for you to retain or achieve Elite membership status for the following calendar year. It sounds as though you finished 2016 with 9 nights and if all of those nights were from 2016, they will rollover into 2017 and you will begin the year with 9 nights. Hope this helps, but please feel free to reach out with any additional questions.
If you ended the year below 10 nights, you would not be eligible to receive any rollover nights. According to the Elite Rollover Nights FAQs Page, to qualify for rollover nights you need to reach at least the Silver Elite level (check the last section on "How does someone reach Marriott Rewards Elite membership status?"). After that, anything above what is needed to retain status (after subtracting previous year's rollover nights) will count towards rollover.