Rollover nights are the nights you earned last above the last level you reached. For example, it takes 50 nights to achieve Gold. If you earned 54 nights last you, you would have Gold status this year and 4 rollover nights. The rollover nights count toward your reward status this year. Going back to the Gold example, with 4 rollover nights, you would only need 46 nights this year to get to Gold.
Be careful though. The year before last 26 nights rolled-over so I started last year with 26 nights. Including these roll-over nights I stayed 152 nights last year so I was expecting to have already 'earned' Platinum the next year. No - when calculating rollover rights they deduct any that were rolled-over form the previous year... so instead of starting the year at 77 nights I started at 51 nights! This effectively discourages you from staying more than 75 nights, including any rollover nights, as they won't count towards next years rollover