Data rollover is a feature typically offered in mobile phone plans, allowing individuals to carry over any leftover data from one month to the next. This ensures that users can preserve their data and avoid wasting it. 

For example, if you have a monthly plan with 10GB of data and have used 7GB, the remaining 3GB will roll over to the next month, giving you a total of 13GB of data in the next cycle, provided the renewal plan is still 10GB. Not all carriers or plans offer this feature, and some may impose limits or expiration dates on rollover data. 

The idea behind data rollover is to provide users with more flexibility and value for what they pay for, helping to prevent the wastage of unused data.