Rollover data is data left over between recharges if you dont use it - you can "rollover" up to 45gb.
It is not extra data so if you are running out of data you will need to recharge.
Information about this is mentioned on our Prepaid Mobile Broadband page under The Details:
Rollover: Up to 50GB when you recharge before expiry. If you accrue over 50GB you will forfeit any data over that limit.