In the majority of cases, we talk about mobile phone plans in months. You pay 'X amount' and the telco gives you 'Y value' for a 30-day period. But in some circumstances your plan's inclusions last longer than just one month and roll over into the next month.
Rollover credit used to be more common on mobile plans, but in the days of 'unlimited' plans and heavy data inclusions, many telcos have reverted to a fixed expiry periods for unused plan inclusions. However, there are still a few exceptions. Rollover credit is most useful when you have varying usage patterns between months. You can store your under-usage of a plan amount in one month and have that provide you with extra usage the following month.