Rollover is transferring from the front-month contract that is near to its expiration date to another contract that is in a further-out month, i.e., taking forward your futures positions. Simply put, this implies that you close your position in the contract that is going to expire and open a similar position in a further-out month contract.

When you buy a future or an option, it will have an expiry date. For instance, a Nifty March 26 future can be only be traded till March 26, 2021. But what if you want to hold the Nifty future till April. In such scenario, you will have to exit the Nifty March and hold a new position in the month of April. This procedure of switching your position from one month to another is known as rollover. 

When you sell the March future, you are liable to pay brokerage charges. Similarly, when you buy back April future, you again have to pay a certain brokerage and charges. These charges are the same as for a normal buy and sell transaction. 

As per a SEBI circular in March 2020, please note that the rollover of contracts during ban periods is not permitted. In case, if you have a position in contract in a ban period, you will be only able to exit the existing position.